Deprecated remove-channel CLI and removeChannel API

Fixes: https://pagure.io/koji/issue/1206

cli/koji_cli/commands.py

@@ -336,6 +336,7 @@ def handle_remove_channel(goptions, session, args):
     parser = OptionParser(usage=get_usage_str(usage))
     parser.add_option("--force", action="store_true", help="force removal, if possible")
     (options, args) = parser.parse_args(args)
+    print("remove-channel is deprecated and will be removed in 1.30")
     if len(args) != 1:
         parser.error("Incorrect number of arguments")
     activate_session(session, goptions)