Makefile: omit building monster placeholder sprites
Makefile: omit building monster placeholder sprites

The placeholder monster sprites (those with the graphic not done
template overlaid) aren't used any more; nothing is linked to them.
Therefore speed up the build process by skipping this step.

This almost compensates for the extra time required to build Freedoom
since the helper scripts were rewritten in Python (joking)

jmtd/freedoom,CWolfRU/freedoom,CWolfRU/freedoom,jmtd/freedoom,jmtd/freedoom