[flang][NFC] Remove hardcoded attr name for fir.global op (#81347)

These hardcoded attribute name are a leftover from the upstreaming
period when there was no way to get the attribute name without an
instance of the operation. It is since possible to do without them and
they should be removed to avoid duplication.

This PR cleanup the fir.global op of these hardcoded attribute name and
use their generated getters. Some other PRs will follow to cleanup other
operations.