[testing] remove unnecessary modification for Android N5X Swarm

chrome_public_test_apk passes now.
https://luci-milo-dev.appspot.com/p/chromium/builders/ci/Android%20N5X%20Swarm/10131

This was introduced in
crrev.com/3400947e

Bug: 955013, 731759
Change-Id: I4b73a7efcf5052d83df66021580a233ece3e66fe
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1993002
Auto-Submit: Takuto Ikuta <tikuta@chromium.org>
Reviewed-by: Yoshisato Yanagisawa <yyanagisawa@google.com>
Commit-Queue: Takuto Ikuta <tikuta@chromium.org>
Cr-Commit-Position: refs/heads/master@{#730109}

testing/buildbot/chromium.swarm.json

@@ -386,7 +386,6 @@
           "--gs-results-bucket=chromium-result-details",
           "--recover-devices"
         ],
-        "experiment_percentage": 100,
         "merge": {
           "args": [
             "--bucket",

testing/buildbot/test_suite_exceptions.pyl

@@ -310,11 +310,6 @@
       'android-code-coverage-native', # https://crbug.com/1018780
     ],
     'modifications': {
-      'Android N5X Swarm': {
-        # TODO(crbug.com/731759): Enable this once we're confident that it
-        # passes.
-        'experiment_percentage': 100,
-      },
       'KitKat Phone Tester (dbg)': {
         'swarming': {
           'shards': 20,