Remove $VERBOSE = nil from tasks.rb

Permanently setting $VERBOSE to nil causes unwanted side effects (warnings generated by app code are
silenced when triggered by a rake task but visible otherwise). silence_warnings {} would be safer to
use here since it resets $VERBOSE back to what it was when the block finishes.
Merge pull request #11601 from TylerRick/dont_permanently_silence_all_warnings

Remove $VERBOSE = nil from tasks.rb