retire SHARED_TOOLCHAIN knob

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