Remove the MODULES_CLEAN variable. It's no longer necessary since

Remove the MODULES_CLEAN variable. It's no longer necessary since
object files for modules are in .po files, while object files for
static use are in .o files. Pointed out by metze.

This reduces the number of files that have to be recompiled after the Makefile
changes. Preventing unnecessary recompiling of the other few is high
on my todo list.
(This used to be commit b9b46d43c7c39d6738c254c4a148bdb93cae02e3)