[XLA:GPU] Delete AnnotateBufferLoadStoreInstructionWithMetadata.

This function was trying to annotate loads/stores to shared memory with
metadata copied from another IRArray.  But it was over-eager and copied
more than we wanted.

In addition, it CHECKed that the source buffer was invariant, and that's
not necessarily true.

For now, remove this function, which is just a performance optimization.

PiperOrigin-RevId: 204005618