Remove custom datalad log handlers while enable library mode

ATM, if there is a root level logger set by Python application, which is a
logical thing to do for an application, then datalad log messages end up
duplicated -- once they are provided by the root level log handler and then by
our very own one.  E.g. this script

	import logging

	## The simplest and documented way to enable logging for a Python app
	logging.basicConfig(level=logging.INFO,
						format='%(name)s : %(asctime)s - %(levelname)s - %(message)s')

	# Alternative way to enable some root level logging
	# which surprised me
	#logging.info('This is an info message')

	import datalad.support.gitrepo as r

	r.lgr.info("datalad logger talking")

	datalad_lgr = logging.getLogger("datalad")

	print(f"logging has handlers: {logging.getLogger().handlers}")
	print(f"DataLad has handlers: {datalad_lgr.handlers}"

would produce following output:

	❯ python simplest_app2.py
	[INFO   ] datalad logger talking
	datalad.gitrepo : 2023-10-24 16:27:02,703 - INFO    - datalad logger talking
	logging has handlers: [<StreamHandler <stderr> (NOTSET)>]
	DataLad has handlers: [<ProgressHandler (NOTSET)>]

so you see our log line duplicated!   With the proposed change, we will
explicitly remove all assigned handlers when enabling library mode, and
if we add to above script the  datalad.enable_librarymode() we get

	❯ python simplest_app2.py
	datalad.gitrepo : 2023-10-24 16:29:25,478 - INFO - datalad logger talking
	logging has handlers: [<StreamHandler <stderr> (NOTSET)>]
	DataLad has handlers: []

We also have to respect config variable so we could handle properly the
DATALAD_RUNTIME_LIBRARYMODE environment variable.

datalad/__init__.py

@@ -81,6 +81,13 @@ def enable_librarymode():
     # export into the environment for child processes to inherit
     os.environ['DATALAD_RUNTIME_LIBRARYMODE'] = '1'
 
+    # disable custom datalad logging handler
+    lgr_handlers = lgr.handlers
+    if lgr_handlers:  # were not removed yet!
+        for h in lgr_handlers:
+            lgr.debug("Enabling library mode: removing datala:wd specific handler %s", h)
+            lgr.removeHandler(h)
+
 
 # For reproducible demos/tests
 _seed = os.environ.get('DATALAD_SEED', None)

datalad/log.py

@@ -621,6 +621,10 @@ def get_initialized_logger(self, logtarget=None):
         -------
         logging.Logger
         """
+        import datalad
+        if 'datalad.runtime.librarymode' in datalad.cfg:
+            return self.lgr
+
         if not logtarget:
             logtarget = self._get_config('target', 'stderr')