MDEV-8437 - plugin variables conflict with bootstrap

Removed redundant attempt to create mysql.plugin table:
- original code was supposed to INSTALL some plugins:
  INSERT INTO plugin VALUES ('innodb', 'ha_innodb.so'),
    ('federated', 'ha_federated.so'), ('blackhole', 'ha_blackhole.so'),
    ('archive', 'ha_archive.so');
- original code was supposed to fail if mysql.plugin exists:
  The query sequence is supposed to be aborted if the CREATE TABLE fails due
  to an already existent table in which case the admin might already have
  chosen to remove one or more plugins.
- mysql.plugin must've been created by preceeding mysql_install_db anyway

debian/mariadb-server-10.1.postinst

@@ -209,14 +209,6 @@ EOF
         "  Show_view_priv='Y', Create_routine_priv='Y', Alter_routine_priv='Y', "\
         "  Create_user_priv='Y', Event_priv='Y', Trigger_priv='Y',"\
         "  ssl_cipher='', x509_issuer='', x509_subject='';"`;
-    # Engines supported by etch should be installed per default. The query sequence is supposed
-    # to be aborted if the CREATE TABLE fails due to an already existent table in which case the
-    # admin might already have chosen to remove one or more plugins. Newlines are necessary.
-    install_plugins=`/bin/echo -e \
-        "USE mysql;\n" \
-        "CREATE TABLE IF NOT EXISTS plugin (name char(64) COLLATE utf8_bin NOT NULL DEFAULT '', " \
-        "  dl char(128) COLLATE utf8_bin NOT NULL DEFAULT '', " \
-        "  PRIMARY KEY (name)) ENGINE=MyISAM DEFAULT CHARSET=utf8 COLLATE=utf8_bin COMMENT='MySQL plugins';" `
 
     # Upgrade password column format before the root password gets set.
     echo "$password_column_fix_query"                        | $MYSQL_BOOTSTRAP 2>&1 | $ERR_LOGGER
@@ -228,7 +220,6 @@ EOF
 
     set +e
     echo "$replace_query"                                    | $MYSQL_BOOTSTRAP 2>&1 | $ERR_LOGGER
-    echo "$install_plugins"                                  | $MYSQL_BOOTSTRAP 2>&1 | $ERR_LOGGER
     set -e
 
     # If there is a real AppArmor profile, we reload it.