[LLDB][CMake][NFC] Remove unused LLDB_LINKER_SUPPORTS_GROUPS variable

The `LLDB_LINKER_SUPPORTS_GROUPS` CMake variable was added back in 2019 as part
of https://reviews.llvm.org/D71306, but it appears to have been unused even
then. This removes the unused variable.

Differential Revision: https://reviews.llvm.org/D156997