[rlgl] Small cleaning

[Bigfoot71]

I simply removed the int format parameter from rlGenTextureMipmaps(), which was unused, as well as the rlTextureFilter enum, which was also unused in rlgl.h and wasn't referenced anywhere else in the library. In any case, TextureFilter replaces it.

[Bigfoot71]

I've just restored the rlTextureFilter enum following our conversation, so the commit only removes the unused int format parameter from rlGenTextureMipmaps.

It's up to you to decide whether it's better to keep this parameter in case it might be reused in the future.

[raysan5]

@asdqwe that's what I meant with my comment but I was not clear enough, format is better to be kept as a parameter, jsut in case.