convert_glib.m4: Remove some unusued conversions
convert_glib.m4: Remove some unusued conversions

These conversions are not used in glibmm. If they are used anywhere else,
they should be defined where they are used.

GNOME/glibmm,GNOME/glibmm,GNOME/glibmm,GNOME/glibmm,GNOME/glibmm