Fix T81081: GPencil preset error

Caused by still having 'texture_opacity' in the preests and the preset
operator (but this was removed in the 2.83 rewrite).

Maniphest Tasks: T81081

Differential Revision: https://developer.blender.org/D8990

release/scripts/startup/bl_operators/presets.py

@@ -686,7 +686,6 @@ class AddPresetGpencilMaterial(AddPresetBase, Operator):
         "gpcolor.texture_offset",
         "gpcolor.texture_scale",
         "gpcolor.texture_angle",
-        "gpcolor.texture_opacity",
         "gpcolor.texture_clamp",
         "gpcolor.mix_factor",
         "gpcolor.show_stroke",