sockets: remove unused KPIs to manipulate sockets

These KPIs were added in dd0e6c383a9f0 and through 15 years had zero use.
They slightly remind what IfAPI does for struct ifnet. But IfAPI does
that for the sake of large collection of NIC drivers not being aware of
struct ifnet. For the sockets it is unclear what could be a large
collection of externally written kernel modules that need extensively use
sockets and not be aware of their internals at the same time. This
isolation of a structure knowledge requires a lot of work, and just
throwing in a few KPIs isn't helpful.

Reviewed by: kib, olce, markj
Differential Revision: https://reviews.freebsd.org/D44311