Remove legacy flipV code in glb-parser

[slimbuck]

Quoting the code comment:

The very first version of FACT generated incorrectly flipped V texture
coordinates. Since this first version was only ever available behind an
editor flag there should be very few such GLB models in the wild.
Instead of bloating the engine forevermore with code to handle this case,
we now issue a warning instead and prompt user to reconvert their FBX.

[willeastcott]

I wholeheartedly approve! 👍