[flang][NFC] Use the tablegen definition for FIR dialect (#84822)

FIROpsDialect has been declared manually with a class inheriting from
the MLIR Dialect class. Another declaration is done using tablegen here
`flang/include/flang/Optimizer/Dialect/FIRDialect.td`. This patch merge
the two declaration so we can use the tablegen generated class for all
the FIROpsDialect needs.

This is part of a series of patch to bring FIR up to date with the
current MLIR infra.