Revert "Enable Crostini experimental UI by default"

This reverts commit a40f79bd.

Reason for revert: This should not have been done. There was a miscommunication between PM and dev.

Original change's description:
> Enable Crostini experimental UI by default
>
> Bug: None
> Test: Shows enabled by default
> Change-Id: I66f99bec91bed541c961835be6cece00da263946
> Reviewed-on: https://chromium-review.googlesource.com/1036475
> Reviewed-by: Nicholas Verne <nverne@chromium.org>
> Commit-Queue: Jeffrey Kardatzke <jkardatzke@google.com>
> Cr-Commit-Position: refs/heads/master@{#554940}

TBR=jkardatzke@google.com,nverne@chromium.org

Change-Id: Ifb82e40345365360f6d40993fd1559ccf9ea1e76
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: None
Reviewed-on: https://chromium-review.googlesource.com/1037646Reviewed-by: James Hawkins <jhawkins@chromium.org>
Commit-Queue: Jeffrey Kardatzke <jkardatzke@google.com>
Cr-Commit-Position: refs/heads/master@{#555192}

chrome/common/chrome_features.cc

@@ -233,7 +233,7 @@ const base::Feature kExperimentalAppBanners {
 
 #if defined(OS_CHROMEOS)
 extern const base::Feature kExperimentalCrostiniUI{
-    "ExperimentalCrostiniUI", base::FEATURE_ENABLED_BY_DEFAULT};
+    "ExperimentalCrostiniUI", base::FEATURE_DISABLED_BY_DEFAULT};
 #endif
 
 // An experimental fullscreen prototype that allows pages to map browser and