Remove some SVN-specific code.

$URL$ is an SVN keyword substitution enabled via
`svn propset svn:keywords "URL" tools/clang/lib/Basic/Version.cpp`.
Now that we no longer use SVN, it's no longer being replaced by
anything, and we no longer offer svn exports. So remove the
$URL$-specific logic.

The "cfe" path prefix removal also no longer makes sense now that
we're on git: Both CLANG_REPOSITORY and LLVM_REPOSITORY are usually
set to https://github.com/llvm/llvm-project.git

So remove that too, and remove the "llvm" prefix removal for symmetry.
With the github url, "llvm" _is_ found in the string, but not in
the place the function expected. Nobody noticed since the llvm
repository path is only used if CLANG_REVISION and LLVM_REVISION are
different, which in the git monorepo world they never should be.
(I might remove the "// Support LLVM in a separate repository"
block in a separate commit.)

Differential Revision: https://reviews.llvm.org/D72848