[Sync] Delete the obselete pref kSyncLongPollIntervalSeconds

This is a follow up to this patch.
http://crrev.com/c/1511975

which made the kSyncLongPollIntervalSeconds pref obsolete.

Bug: 930125
Change-Id: I2566bb2121ea63363cb034e3b0158cf1a437d268
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1518577
Commit-Queue: Mohamed Amir Yosef <[email protected]>
Reviewed-by: Marc Treib <[email protected]>
Cr-Commit-Position: refs/heads/master@{#640272}
diff --git a/chrome/browser/prefs/browser_prefs.cc b/chrome/browser/prefs/browser_prefs.cc
index 027f373..c8f0bf2 100644
--- a/chrome/browser/prefs/browser_prefs.cc
+++ b/chrome/browser/prefs/browser_prefs.cc
@@ -976,4 +976,5 @@
 
   // Added 3/2019.
   syncer::ClearObsoleteFirstSyncTime(profile_prefs);
+  syncer::ClearObsoleteSyncLongPollIntervalSeconds(profile_prefs);
 }