chore: remove unnecessary target platfrom

[pchampio]

fixes: go-flutter-desktop/go-flutter#367

[provokateurin]

Does creating a new main file even make sense then? I mean it just executes the real main function and does nothing else. Most peoply don't change the file so we could just remove it (but still keep backwards compatability).

[pchampio]

I personally like the separation. I agree, this file isn't needed anymore.
WIll have to talk with @GeertJohan , this PR can be a first good starting point for slowly updating the default behavior.

[GeertJohan]

I agree we can move towards removing the main_desktop.dart all together.

If someone needs a separate main file for their destop environment, they could use --target or the hover.yaml target field to set the alternative entrypoint.

Merging this MR already, we can have a separate PR for removing main_dekstop.dart generation and changing the defaults (that will be breaking for people who have custom stuff in their main_desktop.dart).