[CMake] Remove Apple-specific version logic.

We were using the LLDB-Info.plist as the canonical holder of the
version number, but there is really no good reason to do this. If
anything the plist should be generated using the information provided
to CMake.

For now just remove the logic extracting the version from the plist
and rely on LLDB_VERSION_STRING.

llvm-svn: 358604