getentropy: Remove fallback code

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 getrandom syscall.

The getrandom syscall was added in commit e9ac27430c0c ("Implement
getrandom(2) and getentropy(3)") in 2018, and exists in all supported
FreeBSD versions. The ECAPMODE special case applied to a few months
worth of kernel versions also in 2018 -- fixed as of commit ed1fa01ac45a
("Regen after r337998.").

The backwards-compatibility support is no longer needed, so remove it.

Relnotes: Yes
Reviewed by: brooks, cem, delphij
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D47636