Plugin system:

* Remove RNA, Operator and UI for Texture and Sequence plugins.
Since 2.5x no effort has been done to bring that back, so there is simply no reason in keeping that code and the UI for that ;-)
* Low Level code still exists and is unchanged.

source/blender/makesrna/intern/rna_ui.c

@@ -723,7 +723,7 @@ static void rna_def_panel(BlenderRNA *brna)
 
 	prop = RNA_def_property(srna, "bl_context", PROP_STRING, PROP_NONE);
 	RNA_def_property_string_sdna(prop, NULL, "type->context");
-	RNA_def_property_flag(prop, PROP_REGISTER_OPTIONAL); /* should this be optional? - Campbell */
+	RNA_def_property_flag(prop, PROP_REGISTER_OPTIONAL); /* Only used in Properties Editor and 3D View - Thomas */
 	RNA_def_property_ui_text(prop, "Context",
 	                         "The context in which the panel belongs to. (TODO: explain the "
 	                         "possible combinations bl_context/bl_region_type/bl_space_type)");