Makefile: remove "make authors"

Since adding authors involves checking if their contribution requested
exclusion from the AUTHORS list, this isn't an action that should be
run and committed without some manual checking.

Since the script works without arguments and isn't an action most
developers should be running & committing, having a convenience entry
isn't especially useful.

AUTHORS

@@ -17,7 +17,7 @@
 # Please keep the list sorted.
 #
 # The individual authors section can be regenerated using:
-#   make authors
+#   python ./tools/utils/authors_git_gen.py
 
 # BEGIN individuals section.
 Aaron Carlisle <carlisle.aaron00@gmail.com>