Revert "[docs] As of binutils 2.21.51.0.2, ld.bfd supports plugins too, represent this in docs"

This reverts commit r306102.

This change was made without any review, and has a couple of issues.
First, AFAIK we do not test the combination of the LLVM gold plugin with
ld.bfd. Second, the change removed documentation for how to build gold
and replaced it with instructions for building ld.bfd.

llvm-svn: 336841