Revert "Reland "[App Management] Enable App Management by default.""

This reverts commit 8875dfe6.

Reason for revert:
Causes OS settings page to go blank after any search.
http://crbug.com/1003277

Original change's description:
> Reland "[App Management] Enable App Management by default."
> 
> This is a reland of 2fd01309
> 
> Original change's description:
> > [App Management] Enable App Management by default.
> > 
> > Enable App Management by default. Current entry points are about:settings
> > and about:app-management.
> > 
> > Bug: 906508
> > Change-Id: I0118a4b51eda6e65bc72186e771fc2ede2acb45f
> > Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1610658
> > Reviewed-by: Dominick Ng <dominickn@chromium.org>
> > Commit-Queue: Jeevan Shikaram <jshikaram@chromium.org>
> > Cr-Commit-Position: refs/heads/master@{#659341}
> 
> Bug: 906508
> Change-Id: I08def499ce0c87ff44ac722b3bdcd8a238938959
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1782363
> Commit-Queue: Jeevan Shikaram <jshikaram@chromium.org>
> Commit-Queue: Dominick Ng <dominickn@chromium.org>
> Auto-Submit: Jeevan Shikaram <jshikaram@chromium.org>
> Reviewed-by: Dominick Ng <dominickn@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#692990}

TBR=dominickn@chromium.org,jshikaram@chromium.org

# Not skipping CQ checks because original CL landed > 1 day ago.

Bug: 906508, 1003277
Change-Id: Iff5bbb0f33d87bd4931e2f756e21e287af5d6edc
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1803902Reviewed-by: James Cook <jamescook@chromium.org>
Commit-Queue: James Cook <jamescook@chromium.org>
Cr-Commit-Position: refs/heads/master@{#696472}

chrome/common/chrome_features.cc

@@ -680,7 +680,7 @@ const base::Feature kSystemWebApps{"SystemWebApps",
 
 // Enables or disables the App Management UI.
 const base::Feature kAppManagement{"AppManagement",
-                                   base::FEATURE_ENABLED_BY_DEFAULT};
+                                   base::FEATURE_DISABLED_BY_DEFAULT};
 
 // Disable downloads of unsafe file types over insecure transports if initiated
 // from a secure page