sockbuf: remove sbflush_internal() and sbrelease_internal() shims

This functions serve just one purpose - allow to call sbdestroy() from
sofree() without triggering unlocked mutex assertions. Let's just don't
save on locking with INVARIANTS kernel and this will allow to clean up all
these shims. Should be no functional changes.

Reviewed by: markj
Differential Revision: https://reviews.freebsd.org/D49363