Convert plugin searx.plugin.limiter to normal code

The limiter plugin is not a plugin:
* the user can't enable or disable the plugin and there is no point to allow that
* the limiter does use any of the features provides by the plugin framework

This commit convert the limiter plugin into normal code

searx/plugins/limiter.py

@@ -1,38 +0,0 @@
-# SPDX-License-Identifier: AGPL-3.0-or-later
-# lint: pylint
-# pyright: basic
-"""see :ref:`limiter src`"""
-
-import sys
-import flask
-
-from searx import redisdb
-from searx.plugins import logger
-from searx.botdetection import limiter
-
-name = "Request limiter"
-description = "Limit the number of request"
-default_on = False
-preference_section = 'service'
-
-logger = logger.getChild('limiter')
-
-
-def pre_request():
-    """See :ref:`flask.Flask.before_request`"""
-    return limiter.filter_request(flask.request)
-
-
-def init(app: flask.Flask, settings) -> bool:
-    if not settings['server']['limiter'] and not settings['server']['public_instance']:
-        return False
-    if not redisdb.client():
-        logger.error(
-            "The limiter requires Redis, please consult the documentation: "
-            + "https://docs.searxng.org/admin/searx.botdetection.html#limiter"
-        )
-        if settings['server']['public_instance']:
-            sys.exit(1)
-        return False
-    app.before_request(pre_request)
-    return True