[pylint] fix global-variable-not-assigned issues

If there is no write access, there is no need for global.  Remove global
statement if there is no assignment.

global-variable-not-assigned:
  Using global for names but no assignment is done Used when a variable is
  defined through the "global" statement but no assignment to this variable is
  done.

In Pylint 2.11 the global-variable-not-assigned checker now catches global
variables that are never reassigned in a local scope and catches (reassigned)
functions [1][2]

[1] https://pylint.pycqa.org/en/latest/whatsnew/2.11.html
[2] https://github.com/PyCQA/pylint/issues/1375

Signed-off-by: Markus Heiser <markus.heiser@darmarit.de>

searx/network/network.py

@@ -225,12 +225,10 @@ class Network:
 
     @classmethod
     async def aclose_all(cls):
-        global NETWORKS
         await asyncio.gather(*[network.aclose() for network in NETWORKS.values()], return_exceptions=False)
 
 
 def get_network(name=None):
-    global NETWORKS
     return NETWORKS.get(name or DEFAULT_NAME)
 
 
@@ -240,8 +238,6 @@ def initialize(settings_engines=None, settings_outgoing=None):
     from searx import settings
     # pylint: enable=import-outside-toplevel)
 
-    global NETWORKS
-
     settings_engines = settings_engines or settings['engines']
     settings_outgoing = settings_outgoing or settings['outgoing']
 
@@ -328,7 +324,6 @@ def done():
     Note: since Network.aclose has to be async, it is not possible to call this method on Network.__del__
     So Network.aclose is called here using atexit.register
     """
-    global NETWORKS
     try:
         loop = get_loop()
         if loop: