Move validate cleaning from distclean to clean
Move validate cleaning from distclean to clean

This bit me today: I was in validate mode without realising it and "make
clean" didn't help. I don't see a reason for this to be in distclean, as
it isn't generated by ./configure, which is the rule described in
https://ghc.haskell.org/trac/ghc/wiki/Building/Using

Test Plan: Is there a reason for this to be in distclean?

Reviewers: austin, bgamari

Reviewed By: bgamari

Subscribers: rwbarton, thomie

Differential Revision: https://phabricator.haskell.org/D3814

shlevy/ghc,sdiehl/ghc,ezyang/ghc,sdiehl/ghc,shlevy/ghc,shlevy/ghc,sdiehl/ghc,ezyang/ghc,shlevy/ghc,shlevy/ghc,sdiehl/ghc,ezyang/ghc,ezyang/ghc,ezyang/ghc,sdiehl/ghc,sdiehl/ghc,sdiehl/ghc,shlevy/ghc,ezyang/ghc,ezyang/ghc,shlevy/ghc