Make lld cmake not compute commit revision twice

r354605 moved LLD to the unified revision handling introduced in
rL353268 / r352729 and removed uses of LLD_REPOSITORY_STRING and
LLD_REVISION_STRING.

After this change, we no longer compute the (now-unused) values
of these two variables.

Since this removes the only use of llvm/utils/GetRepositoryPath,
remove that too (it's redundant with the system added in r354605).

While here, also remove LLD_VERSION_MAJOR and LLD_VERSION_MINOR.
Their uses were removed in r285163.

Also remove LLD_VERSION from Version.inc which as far as I can
tell has been unused since the file was added in r219277.

No behavior change.

Differential Revision: https://reviews.llvm.org/D72803