Fixes to bugs introduced by PL_stashcache

Fixes to bugs introduced by PL_stashcache
A) Follow suggestion by Benjamin Goldberg to use hv_delete
instead of hv_delete_ent to avoid creating a temporary SV
B) Don't increment the refcount, sneak it into an IV instead
C) When a GP is a deleted that contains a stash, remove the
corresponding entry since hv might be in use in other places.
D) Note that no test cases test the deletion of packages to catch
this bug.

p4raw-id: //depot/perl@19212