[flang][NFC] Remove hardcoded attr name for fir.dispatch and fid.dt_entry ops (#81348)

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.dt_entry and fir.dispatch ops of these hardcoded
attribute name and use their generated getters. Some other PRs will
follow to cleanup other operations.