eliminate legacy function bindings & default binding conflicts

[markwkidd]

In the end, I am a little surprised at just how many mame2003 functions have key binding conflicts with the default RetroArch mapping. Therefore there are a few more changes than I planned, but I've done some testing on my own machine and I think everything's OK.

My two goals:

• Eliminate binding and MAME menu options when they correspond to libretro features that are available
• Change default mame2003 bindings to not conflict with default RetroArch bindings

I think I have made good progress towards both goals, although there still can be more done as we try to address #102

[markwkidd]

@dankcushions this PR hasn't gotten much attention yet. Any chance you would like to give this code a spin to see what you think?

[dankcushions]

I won't be able to test it, but the changes look like they should be ok to me. There was a time when we wouldn't delete this sort of stuff, but instead get rid of it via #ifndef LIBRETRO blocks, but I think that boat may have sailed :)

[markwkidd]

Thanks for the review! If @twinaphex or someone else would like me to use #ifndef it's no problem for me to recreate the PR that way.

As is I took inspiration from the amusing first line of userintrf.c: // __LIBRETRO__: Screw it, this code base isn't being modified elsewhere, just delete what isn't needed :)