retire SHARED_TOOLCHAIN knob

Toolchain components were historically statically linked. They became
normal dynamically linked executables in commit 6ab18ea64d19. There is
no need to keep a special case build option for the toolchain; users who
want statically linked toolchain (or any other) components can use the
existing NO_SHARED knob.

Reviewed by: dim, sjg
Relnotes: Yes
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D41266