bsd.linker.mk: Retire support for binutils 2.17.50

[emaste]

Summary

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") so there is no longer a need
for the build infrastructure.

[emaste]

share/mk/bsd.linker.mk

107–109

I've left these defined unconditionally for now as they may be tested by 3rd party Makefiles. We can remove the in-tree tests for them though, and probably drop them later on.