:beetle: remove missing files from POTFILES

[FineFindus]

Fixes a bug I found when following https://github.com/CleoMenezesJr/escambo/pull/35#issuecomment-1554056491, which caused the PO template generation to file, as src/window.ui seems to longer exist. Removing it from po/POTFILES fixes the error.

ninja: Entering directory `build'
[0/1] Running external command escambo-update-po

xgettext: warning: file 'src/window.ui' extension 'ui' is unknown; will try C
xgettext: error while opening "src/window.ui" for reading: No such file or directory
FAILED: meson-internal__escambo-update-po 

[CleoMenezesJr]

Thanks src/window.ui does not exist anymore, but it's called src/gtk/window.blpnow