[rubygems/rubygems] Simplify and remove some unused code

When `install_with_build_args` was added in
https://github.com/rubygems/rubygems/commit/be96283985cb49c023112117b2ac2dea0d9becf1,
there were two versions of the method: the default version in the base class that still
used the locking `with_build_args`, and an override in the `Future`
class (for Rubygems 2.0 and up) that yielded without calling
`with_build_args`.

The `with_build_args` version of the method was removed in
https://github.com/rubygems/rubygems/commit/8a5b71e3e8072c64a0f3cab838ba330f5e87e37a
while removing a bunch of the old Rubygems compatibility code.

This commit removes `with_build_args`, since it no longer appears to be
used (the build args are passed as a keyword argument to
`spec.source.install` instead, since
https://github.com/rubygems/rubygems/commit/be96283985cb49c023112117b2ac2dea0d9becf1).

The commit also removes `install_with_build_args` and the conditional
around it, since the method wasn't doing anything different than
`install`, and it had a comment that was no longer accurate.

https://github.com/rubygems/rubygems/commit/ba543a60eb

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