Remove PUTBACK/SPAGAIN from hash iter ops

These were added by perl-5a2-2-g463ee0b and perl-5.003-2-gc750a3e.  At
the time, hv_iternext and hv_iterval could reallocate the stack, but
hv_iterkey was safe.  perl-5.6.0-8537-g574c802 divorced the comments
about hv_iterkey from the lines containing hv_iterkey, making things a
bit confusing.  Somewhere along the road, all tied hash functionality
started being guarded with PUSHSTACK, so these hash functions won’t
reallocate the stack any more.
pp.c:pp_quotemeta: No need for PUTBACK

We don’t manipulate the stack pointer at all here.