Revert "Run the layout tests by default on three main windows bots."

This reverts commit 19e20edf8e0882472e71ab6f65e04ffc488ee5be.

Reason for revert: Targets not being built right? See https://build.chromium.org/p/chromium.win/builders/Win%207%20Tests%20x64%20%281%29/builds/29945 

Original change's description:
> Run the layout tests by default on three main windows bots.
> 
> This adds the layout tests to
> - Win7 Tests x64 (1)
> - Win7 Tests (1)
> - Win7 Tests (dbg)(1)
> 
> Which will enable them on the corresponding trybots as well, particularly
> win7_chromium_rel_ng, for all appropriate changes.
> 
> This does not enable this on Win10 Tests x64, which isn't currently supported
> for the layout tests.
> 
> R=​tansell@chromium.org,qyearsley@chromium.org
> NOTRY=true
> BUG=524758
> 
> Change-Id: I9469b5d673bbaad88265b170de72aa7bb205e2b8
> Reviewed-on: https://chromium-review.googlesource.com/719388
> Reviewed-by: Dirk Pranke <dpranke@chromium.org>
> Reviewed-by: Quinten Yearsley <qyearsley@chromium.org>
> Commit-Queue: Dirk Pranke <dpranke@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#508861}

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

Change-Id: I9daf7902e96bfea5371343a69044cba49b0afabe
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: 524758
Reviewed-on: https://chromium-review.googlesource.com/720058
Reviewed-by: Dirk Pranke <dpranke@chromium.org>
Commit-Queue: Dirk Pranke <dpranke@chromium.org>
Cr-Commit-Position: refs/heads/master@{#508898}

• testing/buildbot/chromium.win.json[diff]
• third_party/WebKit/LayoutTests/SlowTests[diff]