Remove old unused "OfflinePagesSvelte" experiment

Bug: 792290
Change-Id: I2321ae02cd36e54f372e22edd74db2253dbc6e9a

Svelte is a term we use for android devices with <= 512MB memory.

We used the OfflinePagesSvelte experiment originally to check if we should
do background offlining on Svelte devices at all.  We determined that we
should, and eventually removed the flag in most places (but not here).
This change removes the flag in the final place.

There is a similarly named experimental flag for doing 'foreground' offlining
on svelte devices - "OfflinePagesSvelteConcurrentLoading".  We plan to enable it
someday, but first we need to fix chromium so that we don't start an additional
process.  Chromium promises not to start more than two processes on android
svelte devices.

Change-Id: I2321ae02cd36e54f372e22edd74db2253dbc6e9a
Reviewed-on: https://chromium-review.googlesource.com/813014Reviewed-by: Jian Li <jianli@chromium.org>
Reviewed-by: Alexei Svitkine <asvitkine@chromium.org>
Commit-Queue: Alexei Svitkine <asvitkine@chromium.org>
Cr-Commit-Position: refs/heads/master@{#522442}

testing/variations/fieldtrial_testing_config.json

@@ -2351,8 +2351,7 @@
                 {
                     "name": "Enabled",
                     "enable_features": [
-                        "OfflinePagesAsyncDownload",
-                        "OfflinePagesSvelte"
+                        "OfflinePagesAsyncDownload"
                     ]
                 }
             ]