Fix logic error in layout translation extraction

Fix logic error in layout translation extraction

lyx_pot.py treats the combination IsPredefined == true and UsesFloatPkg == true
incorrectly: In this case it does not extract the float name, although it needs
to be extracted. This was no problem for LyX 2.0.0, because no layout contained
this combination fo flags. However, the current version of achemso.layout uses
it for several styles, e.g. "List of Schemes".
You only see the bug if you remove lib/layouttranslations before updating it,
otherwise it would be covered by the old translations which are kept. The fix
make use of the fact that IsPredefined == false and UsesFloatPkg == false is
not supported by LyX to simplify the logic.