gst/gststructure.c: Remove that ugly if-then thing in the code that converts between...

gst/gststructure.c: Remove that ugly if-then thing in the code that converts between strings and types.

Original commit message from CVS:
* gst/gststructure.c: (gst_structure_get_abbrs),
(gst_structure_from_abbr), (gst_structure_to_abbr):
Remove that ugly if-then thing in the code that converts
between strings and types.