[po] Remove gmathmlconvert.c in POTFILES.
[po] Remove gmathmlconvert.c in POTFILES.

Fixes make check.

wavewave/lasem,gjtorikian/lasem,gjtorikian/lasem,gjtorikian/lasem,janmarthedal/mathml2svg,gjtorikian/lasem,janmarthedal/mathml2svg,janmarthedal/mathml2svg,wavewave/lasem,wavewave/lasem,janmarthedal/mathml2svg,wavewave/lasem