use staticcheck instead of megacheck

[weekface]

What problem does this PR solve?

https://github.com/dominikh/go-tools/blob/5a4a2f4a438d01ba03c591f88ef312005a05063b/doc/2019.1.html

At the core of the 2019.1 release lies the grand restructuring of all of the staticcheck tools.
  All of the individual checkers, as well as megacheck, have been merged into a single tool,
  which is simply called staticcheck.

So we use staticcheck instead of megacheck.

This pr fixes: #546

What is changed and how it works?

Check List

Tests

• No code

Code changes

• Has CI related scripts change

Side effects

Related changes

Does this PR introduce a user-facing change?:

NONE

[weekface]

/run-e2e-tests