retire SHARED_TOOLCHAIN knob

Toolchain components were historically statically linked.  They became
normal dynamically linked executables in commit 6ab18ea64d.  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

share/mk/local.sys.dirdeps.mk

@@ -64,9 +64,6 @@ WITH_META_STATS= t
 .endif
 
 # toolchains can be a pain - especially bootstrappping them
-.if ${MACHINE} == "host"
-MK_SHARED_TOOLCHAIN= no
-.endif
 TOOLCHAIN_VARS=	AS AR CC CLANG_TBLGEN CXX CPP LD NM OBJCOPY RANLIB \
 		STRINGS SIZE LLVM_TBLGEN
 _toolchain_bin_CLANG_TBLGEN=	/usr/bin/clang-tblgen

share/mk/src.opts.mk

@@ -168,7 +168,6 @@ __DEFAULT_YES_OPTIONS = \
     SENDMAIL \
     SERVICESDB \
     SETUID_LOGIN \
-    SHARED_TOOLCHAIN \
     SHAREDOCS \
     SOURCELESS \
     SOURCELESS_HOST \