[mlir][EmitC] Remove `func.constant` from emitter (#82342)

As part of the renaming the Standard dialect to Func dialect, *support*
for the `func.constant` operation was added to the emitter. However, the
emitter cannot emit function types. Hence the emission for a snippet
like

```
%0 = func.constant @myfn : (f32) -> f32

func.func private @myfn(%arg0: f32) -> f32 {
  return %arg0 : f32
}
```

failes with `func.mlir:1:6: error: cannot emit type '(f32) -> f32'`.
This removes `func.constant` from the emitter.