gbde: stop rebuilding sha512 bits

In the early days of gbde, it linked against libmd. Shortly after
conception, phk replaced ARC4 with SHA-512, but libmd did not have SHA2
at the time thus he built a copy of sha2.c for gbde.

Fast forward 3 years, cperciva adds SHA2 to libmd -- this makes gbde's
build of sha2.c redundant, but it's (understandably) overlooked. Let's
simplify the gbde build now and just assume that libmd includes the most
optimal implementation.

Reported by: koobs (weird lto errors?)
Differential Revision: https://reviews.freebsd.org/D34668