Revert "Set is_try_builder on mac10.13_blink_rel in builders.json."

This reverts commit 47b21786.

Reason for revert: rebaseline-cl fails, which blocks wpt-import

It's a bit premature to set this flag, which includes mac10.13 in
rebaseline-cl automatically. Baselines haven't been created for mac10.13
so the first engineer/bot that tries to run `rebaseline-cl` with the
flag enabled will need to conduct the huge mac10.13 rebaseline.

Original change's description:
> Set is_try_builder on mac10.13_blink_rel in builders.json.
> 
> To optimize baselines properly, rebaseline-cl needs mac10.13_blink_rel
> in its try bot set, because mac10.13 is the only port whose baseline
> search path begins with "platform/mac".  If BaselineOptimizer doesn't
> see a baseline in platform/mac, it will consolidate matching baselines
> from other Mac ports under platform/mac-mac10.12, and fail to merge them
> with matching baselines in platform/win into the generic path.
> 
> Bug: 812784, 774301
> Change-Id: Ifda7a0051bed79a044ab516b40c0795242247b46
> Reviewed-on: https://chromium-review.googlesource.com/922791
> Commit-Queue: Steve Kobes <skobes@chromium.org>
> Reviewed-by: Dirk Pranke <dpranke@chromium.org>
> Reviewed-by: Quinten Yearsley <qyearsley@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#537146}

TBR=qyearsley@chromium.org,dpranke@chromium.org,skobes@chromium.org

Change-Id: Id6c23f2bde53ac42fbc49fa5b5211b2f1d90303f
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: 812784, 774301
Reviewed-on: https://chromium-review.googlesource.com/923781Reviewed-by: Robert Ma <robertma@chromium.org>
Commit-Queue: Robert Ma <robertma@chromium.org>
Cr-Commit-Position: refs/heads/master@{#537244}

third_party/WebKit/Tools/Scripts/webkitpy/common/config/builders.json

@@ -70,8 +70,7 @@
     },
     "mac10.13_blink_rel": {
         "port_name": "mac-mac10.13",
-        "specifiers": ["Mac10.13", "Release"],
-        "is_try_builder": true
+        "specifiers": ["Mac10.13", "Release"]
     },
     "win7_blink_rel": {
         "port_name": "win-win7",