Remove useless python3 FLAVOR from devel/py-backports-abc

Here's another removal of a useless python3 FLAVOR in a port that
backports functionality.

The only port that uses this is py-tornado, and it doesn't use the
python3 flavor.

OK?

--Kurt

Index: Makefile
===================================================================
RCS file: /cvs/ports/devel/py-backports-abc/Makefile,v
retrieving revision 1.5
diff -u -p -r1.5 Makefile
--- Makefile 28 Apr 2019 20:51:30 -0000 1.5
+++ Makefile 1 May 2019 04:21:52 -0000
@@ -19,8 +19,7 @@ MODULES = lang/python
MODPY_PI = Yes
MODPY_SETUPTOOLS = Yes

-FLAVORS = python3
-FLAVOR ?=
+# No python3 FLAVOR - port backports functionality to python 2.x

do-test:
cd ${WRKSRC} && ${MODPY_BIN} -m unittest discover