fix: Remove beam-master build

Author: untitaker

tox.ini

@@ -32,7 +32,7 @@ envlist =
     {pypy,py2.7,py3.5,py3.6,py3.7,py3.8}-celery-{4.1,4.2,4.3}
     {pypy,py2.7}-celery-3
 
-    {py2.7,py3.6}-beam-{12,13,master}
+    {py2.7,py3.6}-beam-{12,13}
     py3.7-beam-{12,13}
 
     # The aws_lambda tests deploy to the real AWS and have their own matrix of Python versions.
@@ -98,7 +98,6 @@ deps =
 
     beam-12: apache-beam>=2.12.0, <2.13.0
     beam-13: apache-beam>=2.13.0, <2.14.0
-    beam-master: git+https://github.com/apache/beam#egg=apache-beam&subdirectory=sdks/python
 
     celery-3: Celery>=3.1,<4.0
     celery-4.1: Celery>=4.1,<4.2