gem: Remove kindlegen from Gemfile

KindleGen is unbuildable now as binaries are no longer available for
download.

This commit disables the Kindle autogeneration.

Refs: tdtds/kindlegen#42
Signed-off-by: Otavio Salvador <otavio@ossystems.com.br>

Gemfile

@@ -13,4 +13,4 @@ gem 'coderay'
 gem 'pygments.rb'
 gem 'thread_safe'
 gem 'epubcheck'
-gem 'kindlegen'
+

Gemfile.lock

@@ -37,9 +37,6 @@ GEM
       rubyzip (> 1.1.1, < 2.3)
     hashery (2.1.2)
     json (2.3.1)
-    kindlegen (3.0.5)
-      rake
-      rubyzip
     mime-types (3.3.1)
       mime-types-data (~> 3.2015)
     mime-types-data (3.2020.0512)
@@ -91,7 +88,6 @@ DEPENDENCIES
   coderay
   epubcheck
   json
-  kindlegen
   pygments.rb
   rake
   thread_safe

Rakefile

@@ -32,7 +32,7 @@ namespace :build do
 end
 
 desc 'Build all default formats'
-task :build => [ "build:html", "build:epub", "build:mobi", "build:pdf" ]
+task :build => [ "build:html", "build:epub", "build:pdf" ]
 
 desc 'Default task'
 task :default => [ "build" ]