setup: file_finders entry-point group registration is not a workaround

As discussion in https://bitbucket.org/pypa/setuptools/issue/313
unveiled, setuptools do not have to carry setuptools.file_finders
entrypoint with it in order to support _other_ projects to use
.file_finders entry point.

In our case it means that is is normal that we have to make sure that
the group we are going to register entry-point into, exists.

Remove erroneous comments introduced in 11d130d1 (setup: Ensure
setuptools.file_finders entry-point group is registered).

setup.py

@@ -146,13 +146,6 @@ def register_as_entrypoint(func, entryname, groupname, distname):
     from pkg_resources import working_set, EntryPoint
     dist = working_set.by_key[distname]
 
-    # workaround for setuptools 9.0 dropping `setuptools.file_finders` entrypoint
-    # group registration:
-    #   https://bitbucket.org/pypa/setuptools/commits/f191c8a1225bd58a5fb5aa9abb31b06dc710f0b9#Lsetup.pyF175
-    #   https://bitbucket.org/pypa/setuptools/issue/313
-    # issue reported back:
-    #   https://bitbucket.org/pypa/setuptools/issue/313#comment-18430008
-    #
     # register group if it is not yet registered
     # else dist.get_entry_map(groupname) returns just {} not connected to entry map
     entry_map = dist.get_entry_map()