bsd.linker.mk: Retire support for binutils 2.17.50

GNU binutils 2.17.50 is the last GPLv2 version, and we retained support
in FreeBSD while it was part of the in-tree toolchain. The last parts
of binutils 2.17.50 were removed in commit 74e8d41e0ac8 ("Retire
BINUTILS and BINUTILS_BOOTSTRAP options") and there is no longer a need
for the build infrastructure.

We retain build-id and ifunc in LINKER_FEATURES unconditionally for now,
as they may be tested by 3rd-party Makefiles. We can however stop
testing for them in the tree.

Reviewed by: imp
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D55674