Move ComputeEffectiveMap code and avoid some duplication

Move ComputeEffectiveMap code and avoid some duplication

The ComputeEffectiveMap function is only called from keytypes.c, with
the last argument NULL, so we can move it there and remove some code.
The function XkbcVirtualModsToRealMods, of which the above is the only
user, is already implemented more simply in compat.c, so make this one
non-static and use it. This leaves src/xkb.c empty, so remove it.

Signed-off-by: Ran Benita <ran234@gmail.com>