In warnings.pm, delete a hash slice, instead of using a loop.

In warnings.pm, delete a hash slice, instead of using a loop.

Deleting a hash slice compiles 5 fewer ops, and executes 21 fewer than
looping over the keys to delete each in turn. Whilst this is arguably a
micro-optimisation, it does not increase obfuscation and is in code loaded
by nearly every Perl program, so feels worthwhile.