Revert "Introduce sharding in memory.leak_detection"

This reverts commit b621992.

Reason for revert:
memory.leak_detection failing on chromium.linux/Leak Detection Linux

Builders failed on: 
- Leak Detection Linux: 
  https://ci.chromium.org/p/chromium/builders/luci.chromium.ci/Leak%20Detection%20Linux

Original change's description:
> Introduce sharding in memory.leak_detection
> 
> This CL expands the page_set of memory.leak_detection to 200 and introduces sharding.
> 
> Cq-Include-Trybots: master.tryserver.chromium.perf:obbs_fyi
> Change-Id: I6d562fcc80df8089887236363c7d5b0fdf2b879e
> Reviewed-on: https://chromium-review.googlesource.com/1180788
> Reviewed-by: John Budorick <jbudorick@chromium.org>
> Reviewed-by: Ned Nguyen <nednguyen@google.com>
> Commit-Queue: Ned Nguyen <nednguyen@google.com>
> Cr-Commit-Position: refs/heads/master@{#584476}

TBR=dpranke@chromium.org,nednguyen@google.com,martiniss@chromium.org,yuzus@chromium.org,bpastene@chromium.org,jbudorick@chromium.org

Change-Id: Ie42a59433e4d133b5cc467cafd694224bd114391
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Cq-Include-Trybots: master.tryserver.chromium.perf:obbs_fyi
Reviewed-on: https://chromium-review.googlesource.com/1181961
Reviewed-by: apacible <apacible@chromium.org>
Commit-Queue: apacible <apacible@chromium.org>
Cr-Commit-Position: refs/heads/master@{#584512}

testing/buildbot/chromium.linux.json

@@ -801,7 +801,6 @@
         "args": [
           "--benchmarks=memory.leak_detection",
           "--smoke-test-mode",
-          "--test-shard-map-filename=linux_leak_detection_shard_map.json",
           "--upload-results",
           "--output-format=histograms",
           "--browser=release",
@@ -823,8 +822,7 @@
           "expiration": 36000,
           "hard_timeout": 10800,
           "ignore_task_failure": false,
-          "io_timeout": 3600,
-          "shards": 10
+          "io_timeout": 3600
         }
       }
     ]

testing/buildbot/test_suites.pyl

@@ -1619,7 +1619,6 @@
       'args': [
         '--benchmarks=memory.leak_detection',
         '--smoke-test-mode',
-        '--test-shard-map-filename=linux_leak_detection_shard_map.json',
         '--upload-results',
         '--output-format=histograms',
         '--browser=release',
@@ -1640,7 +1639,6 @@
         'hard_timeout': 10800,
         'ignore_task_failure': False,
         'io_timeout': 3600,
-        'shards': 10
       },
     },
   },