[android] Remove custom timeout from M phone tester.

Some builds timed out recently:
https://ci.chromium.org/p/chromium/builders/luci.chromium.ci/Marshmallow%2064%20bit%20Tester

Bug: 874840
Change-Id: I31aa161ef63488debb8c3fe38f71c096fe132207
Reviewed-on: https://chromium-review.googlesource.com/1177703
Commit-Queue: Arthur Sonzogni <arthursonzogni@chromium.org>
Reviewed-by: John Budorick <jbudorick@chromium.org>
Cr-Commit-Position: refs/heads/master@{#583669}

testing/buildbot/waterfalls.pyl

@@ -293,7 +293,6 @@
               'os': 'Android',
             },
           ],
-          'hard_timeout': 960,
         },
         'os_type': 'android',
       },