Blender Git Loki

Fix T66369: Excessive WARN messages in console when opening older files

CDData checking on file load was not taking into account deprecated
CD_MTEXPOLY datatype, which unfortunately shows same weird glitch as
CD_PAINT_MASK and CD_FACEMAP ones...

Note that it was annoying (due to amount of warnings in console), but
totally harmless, since that data type is just deleted anyway.

This commit also generally cleans up the CD_MTEXPOLY deprecation code, we
have a system to handle that, let's use it, instead of defining local
static values to replace it...