GPv3: Remove unused legacy editor functions for GPv2

Removes all unused functions in ED_gpencil_legacy.hh

Pull Request: https://projects.blender.org/blender/blender/pulls/128597

doc/python_api/sphinx_doc_gen.py

@@ -1205,8 +1205,6 @@ context_type_map = {
     "edit_object": [("Object", False)],
     "edit_text": [("Text", False)],
     "editable_bones": [("EditBone", True)],
-    "editable_gpencil_layers": [("GPencilLayer", True)],
-    "editable_gpencil_strokes": [("GPencilStroke", True)],
     "editable_objects": [("Object", True)],
     "editable_fcurves": [("FCurve", True)],
     "fluid": [("FluidSimulationModifier", False)],