[rlgl] Remove unused 'format' parameter in rlGenTextureMipmaps

[Not-Nik]

Unless I had a major oversight somewhere, that parameter isn't used, so let's leave it out entirely.

[Bigfoot71]

I had already proposed it some time ago and the final opinion is that it was better to keep it
See: #4583

About the format parameter in rlGenTextureMipmaps(), it was kept there for a future implementation for OpenGL 1.1 that required the data size in memory... but implementation was removed at some point in raylib live...

[Not-Nik]

So we had a 1.1 implementation, then it was removed and now we're waiting for something to happen so we can add it again? Regardless, unless there's solid plans to add a 1.1 implementation, I reckon it would be OK to just remove the parameter(s) for now and add them again once the implementation is actually there.

[raysan5]

As commented, we are keeping this parameter just in case it can be required on a future implementation (OpenGL 1.1, software render, old consoles...)