Remove 'ensure previews' call from write code.

Sorry about that, should have checked this stuff more, with Internal material
renders are very fast (unoticable), but with Cycles it can take (a lot of) time,
like several minutes or more.

Will probably fall back to a dedicated operator users will have to fire themselves
when they want previews in their files.

source/blender/windowmanager/intern/wm_files.c

@@ -957,8 +957,6 @@ int wm_file_write(bContext *C, const char *filepath, int fileflags, ReportList *
 	/* save before exit_editmode, otherwise derivedmeshes for shared data corrupt #27765) */
 	if ((U.flag & USER_SAVE_PREVIEWS) && BLI_thread_is_main()) {
 		ibuf_thumb = blend_file_thumb(CTX_data_scene(C), CTX_wm_screen(C), &thumb);
-
-		wm_ensure_previews(C, G.main);
 	}
 
 	BLI_callback_exec(G.main, NULL, BLI_CB_EVT_SAVE_PRE);