kern: random: drop read_rate and associated functionality

Refer to discussion in PR 230808 for a less incomplete discussion, but
the gist of this change is that we currently collect orders of magnitude
more entropy than we need.

The excess comes from bytes being read out of /dev/*random. The default
rate at which we collect entropy without the read_rate increase is
already more than we need to recover from a compromise of an internal
state.

Reviewed by: csprng (cem, delphij, markm)
Differential Revision: https://reviews.freebsd.org/D32021