pdb, libgimp: remove _gimp_register_load_handler()

and rename _gimp_register_magic_load_handler() to
_gimp_register_load_handler().
3 jobs for master in 14 minutes and 59 seconds (queued for 1 second)
Status Job ID Name Coverage
  Babl
passed #426813
babl-git

00:02:10

 
  Gegl
passed #426814
gegl-git

00:03:40

 
  Gimp
passed #426815
build-git-autotools

00:09:08