Plugins: Clean up uninstall_plugins option during database upgrade.

`register_uninstall_hook()` is designed to be given a function callback (or a static class variable), not an array/object instance. This got blocked in [16339] but the option itself was never cleaned up.

Props polevaultweb.
Fixes #31625.
Built from https://develop.svn.wordpress.org/trunk@37965


git-svn-id: http://core.svn.wordpress.org/trunk@37906 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/version.php

@@ -4,14 +4,14 @@
  *
  * @global string $wp_version
  */
-$wp_version = '4.6-beta1-37964';
+$wp_version = '4.6-beta1-37965';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.
  *
  * @global int $wp_db_version
  */
-$wp_db_version = 37854;
+$wp_db_version = 37965;
 
 /**
  * Holds the TinyMCE version