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.

railties/lib/rails/tasks.rb

@@ -1,5 +1,3 @@
-$VERBOSE = nil
-
 # Load Rails Rakefile extensions
 %w(
   annotations