getentropy: Remove fallback code

[emaste]

Summary

We don't in general support running newer libc on an older kernel, but
have occasionally added support for specific functionality on a case-by-
case basis.  When we do this it is usually done as an aid for developers
to get across a change that introduced new functionality, as for 64-bit
inodes and the introduction of the getentropy syscall.

The getentropy syscall was added in commit ed1fa01ac45a ("Regen after
r337998.") in 2018, and exists in all supported FreeBSD versions.  The
backwards-compatibility support is no longer needed, so remove it.

Sponsored by:   The FreeBSD Foundation

[cem]

[emaste]

In the commit message I'll add a note that the special case for handling ECAPMODE applied to a few months worth of kernel versions also in 2018.