Remove GPU pool requirement for SKGL bot

Previously, the SkiaRenderer GL bot was added with the restriction
that it must find matching devices in the GPU pool. But the GPU
pool did not include any Nexus 5Xs which could satisfy the
SkiaRenderer GL bot's other requirements.

This CL removes the GPU pool requirement for the SkiaRenderer GL
bot.

Bug: 969855
Change-Id: I69787e0415890c6e5755df6cfb3be99b4de92c3f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1796642Reviewed-by: Brian Sheedy <bsheedy@chromium.org>
Reviewed-by: John Budorick <jbudorick@chromium.org>
Commit-Queue: Chris Blume <cblume@chromium.org>
Cr-Commit-Position: refs/heads/master@{#695290}

testing/buildbot/chromium.gpu.fyi.json

@@ -6911,8 +6911,7 @@
               "device_os": "MMB29Q",
               "device_os_type": "userdebug",
               "device_type": "bullhead",
-              "os": "Android",
-              "pool": "Chrome-GPU"
+              "os": "Android"
             }
           ],
           "idempotent": false
@@ -6942,8 +6941,7 @@
               "device_os": "MMB29Q",
               "device_os_type": "userdebug",
               "device_type": "bullhead",
-              "os": "Android",
-              "pool": "Chrome-GPU"
+              "os": "Android"
             }
           ],
           "idempotent": false
@@ -6973,8 +6971,7 @@
               "device_os": "MMB29Q",
               "device_os_type": "userdebug",
               "device_type": "bullhead",
-              "os": "Android",
-              "pool": "Chrome-GPU"
+              "os": "Android"
             }
           ],
           "idempotent": false
@@ -7004,8 +7001,7 @@
               "device_os": "MMB29Q",
               "device_os_type": "userdebug",
               "device_type": "bullhead",
-              "os": "Android",
-              "pool": "Chrome-GPU"
+              "os": "Android"
             }
           ],
           "idempotent": false
@@ -7042,8 +7038,7 @@
               "device_os": "MMB29Q",
               "device_os_type": "userdebug",
               "device_type": "bullhead",
-              "os": "Android",
-              "pool": "Chrome-GPU"
+              "os": "Android"
             }
           ],
           "idempotent": false
@@ -7074,8 +7069,7 @@
               "device_os": "MMB29Q",
               "device_os_type": "userdebug",
               "device_type": "bullhead",
-              "os": "Android",
-              "pool": "Chrome-GPU"
+              "os": "Android"
             }
           ],
           "idempotent": false
@@ -7121,8 +7115,7 @@
               "device_os": "MMB29Q",
               "device_os_type": "userdebug",
               "device_type": "bullhead",
-              "os": "Android",
-              "pool": "Chrome-GPU"
+              "os": "Android"
             }
           ],
           "idempotent": false,

testing/buildbot/waterfalls.pyl

@@ -2571,7 +2571,6 @@
         'browser_config': 'android-chromium',
         'skip_merge_script': True,
         'mixins': [
-          'gpu_pool',
           'marshmallow',
           'bullhead',
        ],