ds: favor `delete' over assigning fields to `undef'

ds: favor `delete' over assigning fields to `undef'

This is cleaner in most cases and may allow Perl to reuse memory
from unused fields.

We can do this now that we no longer support Perl 5.8; since
Danga::Socket was written with struct-like pseudo-hash support
in mind, and Perl 5.9+ dropped support for pseudo-hashes over
a decade ago.