Blender Git Loki

Cleanup/refactor: Remove logically broken code from GPencil undo.

`ED_undo_gpencil_step` only support valid undo step direction, passing
step name here is useless and only add confusion to what works or not.

Undo by step name or step index is fully not supported by GPencil undo
mode currently.

Note that since GPencil undo mode does not seem to ever be used anyway,
this is not an urgent issue in practice, but this needs to be cleaned up
at some point. See also T84703.