It was rather a huge chunk of code, which started to become
more harder to maintain with the transition to OpenSubdiv based
implementation. Because of this transition, the compatibility was
also rather on a poor side.
Remove compatibility support for pre-2.50.9 multires.
Ref T77107
Reviewed By: brecht, mont29
Differential Revision: https://developer.blender.org/D9238
d11e357824