MDL-77988 dml: correct logic when dropping plugin tables.

Comparison of method void return within a conditional didn't make
sense, and the aforementioned method throws an exception on error
anyway which would halt execution.

lib/adminlib.php

@@ -370,7 +370,8 @@ function drop_plugin_tables($name, $file, $feedback=true) {
     global $CFG, $DB;
 
     // first try normal delete
-    if (file_exists($file) and $DB->get_manager()->delete_tables_from_xmldb_file($file)) {
+    if (file_exists($file)) {
+        $DB->get_manager()->delete_tables_from_xmldb_file($file);
         return true;
     }