Remove the SYMVER build option.

This option was added as a transition aide when symbol versioning was
first added. It was enabled by default in 2007 and is supported even
by the old GPLv2 binutils. Trying to disable it currently fails to
build in libc and at this point it isn't worth fixing the build.

Reported by: Michael Dexter
Reviewed by: imp
Differential Revision: https://reviews.freebsd.org/D24637