Disable several ignore-warning flags in genapply.

This commit is:

• individually buildable
• has commit messages which describing what it does
• [n/a] have added source comments describing your change. Warning fixes - no major code changes.
• [n/a] add a testcase to the testsuite. - Warning fixes - no code added except an Impossible case.
• Several flags were enabled in utils/genapply/Main.hs to disable several warnings. As per the message, I have fixed the cause of the warnings and removed the disabled flags.

Apologies if there is anything erroneous in this commit - This is my first PR to GHC.