Remove simplecov and simplecov-rcov tentatively

to see if this argument error while `Fetching simplecov-rcov 0.3.5` is addressed:

https://github.com/yahonda/httpclient/actions/runs/7695505773/job/20968521867#step:3:72

```
ArgumentError: wrong number of arguments (given 4, expected 1)
  /home/runner/work/httpclient/httpclient/vendor/bundle/ruby/2.5.0/gems/psych-5.1.2/lib/psych.rb:322:in `safe_load'
  /opt/hostedtoolcache/Ruby/2.5.9/x64/lib/ruby/2.5.0/rubygems/safe_yaml.rb:31:in `safe_load'
  /opt/hostedtoolcache/Ruby/2.5.9/x64/lib/ruby/2.5.0/rubygems/package.rb:496:in `block (2 levels) in read_checksums'
  /opt/hostedtoolcache/Ruby/2.5.9/x64/lib/ruby/2.5.0/rubygems/package.rb:495:in `wrap'
  /opt/hostedtoolcache/Ruby/2.5.9/x64/lib/ruby/2.5.0/rubygems/package.rb:495:in `block in read_checksums'
  /opt/hostedtoolcache/Ruby/2.5.9/x64/lib/ruby/2.5.0/rubygems/package/tar_reader.rb:116:in `seek'
  /opt/hostedtoolcache/Ruby/2.5.9/x64/lib/ruby/2.5.0/rubygems/package.rb:494:in `read_checksums'
  /opt/hostedtoolcache/Ruby/2.5.9/x64/lib/ruby/2.5.0/rubygems/package.rb:547:in `block (2 levels) in verify'
  /opt/hostedtoolcache/Ruby/2.5.9/x64/lib/ruby/2.5.0/rubygems/package/tar_reader.rb:29:in `new'
  /opt/hostedtoolcache/Ruby/2.5.9/x64/lib/ruby/2.5.0/rubygems/package.rb:546:in `block in verify'
  /opt/hostedtoolcache/Ruby/2.5.9/x64/lib/ruby/2.5.0/rubygems/package/file_source.rb:30:in `open'
  /opt/hostedtoolcache/Ruby/2.5.9/x64/lib/ruby/2.5.0/rubygems/package/file_source.rb:30:in `with_read_io'
  /opt/hostedtoolcache/Ruby/2.5.9/x64/lib/ruby/2.5.0/rubygems/package.rb:545:in `verify'
  /opt/hostedtoolcache/Ruby/2.5.9/x64/lib/ruby/2.5.0/rubygems/package.rb:526:in `spec'
  /opt/hostedtoolcache/Ruby/2.5.9/x64/lib/ruby/gems/2.5.0/gems/bundler-2.3.27/lib/bundler/rubygems_gem_installer.rb:95:in `spec'
  /opt/hostedtoolcache/Ruby/2.5.9/x64/lib/ruby/gems/2.5.0/gems/bundler-2.3.27/lib/bundler/source/rubygems.rb:190:in `install'
  /opt/hostedtoolcache/Ruby/2.5.9/x64/lib/ruby/gems/2.5.0/gems/bundler-2.3.27/lib/bundler/installer/gem_installer.rb:54:in `install'
  /opt/hostedtoolcache/Ruby/2.5.9/x64/lib/ruby/gems/2.5.0/gems/bundler-2.3.27/lib/bundler/installer/gem_installer.rb:16:in `install_from_spec'
  /opt/hostedtoolcache/Ruby/2.5.9/x64/lib/ruby/gems/2.5.0/gems/bundler-2.3.27/lib/bundler/installer/parallel_installer.rb:186:in `do_install'
  /opt/hostedtoolcache/Ruby/2.5.9/x64/lib/ruby/gems/2.5.0/gems/bundler-2.3.27/lib/bundler/installer/parallel_installer.rb:177:in `block in worker_pool'
  /opt/hostedtoolcache/Ruby/2.5.9/x64/lib/ruby/gems/2.5.0/gems/bundler-2.3.27/lib/bundler/worker.rb:62:in `apply_func'
  /opt/hostedtoolcache/Ruby/2.5.9/x64/lib/ruby/gems/2.5.0/gems/bundler-2.3.27/lib/bundler/worker.rb:57:in `block in process_queue'
  /opt/hostedtoolcache/Ruby/2.5.9/x64/lib/ruby/gems/2.5.0/gems/bundler-2.3.27/lib/bundler/worker.rb:54:in `loop'
  /opt/hostedtoolcache/Ruby/2.5.9/x64/lib/ruby/gems/2.5.0/gems/bundler-2.3.27/lib/bundler/worker.rb:54:in `process_queue'
  /opt/hostedtoolcache/Ruby/2.5.9/x64/lib/ruby/gems/2.5.0/gems/bundler-2.3.27/lib/bundler/worker.rb:91:in `block (2 levels) in create_threads'

```

Gemfile

@@ -10,8 +10,6 @@ group :development do
   gem 'rake', :require => false
   gem 'rdoc'
   gem 'test-unit'
-  gem 'simplecov'
-  gem 'simplecov-rcov'
   gem 'pry'
   gem 'rack', '~> 2.2'
   gem 'rubysspi'

test/helper.rb

@@ -1,11 +1,4 @@
 # -*- encoding: utf-8 -*-
-begin
-  require 'simplecov'
-  require 'simplecov-rcov'
-  SimpleCov.formatter = SimpleCov::Formatter::RcovFormatter
-  SimpleCov.start
-rescue LoadError
-end
 require 'test/unit'
 
 require 'httpclient'