MDL-78552 core: tidy up MDL_PERF constants

* all constans usable in ABORT_AFTER_CONFIG should be always defined
* MDL_PERFDB and $PERF->logwrites not used after legacy log removal
* MDL_PERF_TEST should be documented in codebase
* deprecated warnings in shutdowb manager

lib/classes/task/logmanager.php

@@ -301,7 +301,7 @@ class logmanager {
                 self::$logpath,
                 $failed,
                 $DB->perf_get_reads() - self::$taskloginfo->dbread,
-                $DB->perf_get_writes() - self::$taskloginfo->dbwrite - $PERF->logwrites,
+                $DB->perf_get_writes() - self::$taskloginfo->dbwrite,
                 self::$taskloginfo->timestart,
                 microtime(true)
             );