Update GimpExportProcedure Parameters (Part 1)
Completing the first step of #7370 (comment 2096869) by removing the n_drawables and drawables parameters from GimpExportProcedure. Also, removes the now unnecessary GIMP_EXPORT_CANCEL enum (defaults to GIMP_EXPORT_IGNORE as some plug-ins already did).
Marked as a draft because I need to test exporting in all affected formats.