Remove DiskCache.TrimDelays histogram.

This histogram is not defined in histograms.xml and is not being used.

Bug: 643561
Change-Id: Ib85c40e3f5df88df51caba61f9888e887219ee83
Reviewed-on: https://chromium-review.googlesource.com/1055951Reviewed-by: Maks Orlovich <morlovich@chromium.org>
Commit-Queue: Chris Bentzel <cbentzel@chromium.org>
Cr-Commit-Position: refs/heads/master@{#558036}

net/disk_cache/blockfile/eviction.cc

@@ -240,7 +240,6 @@ bool Eviction::ShouldTrim() {
     return false;
   }
 
-  UMA_HISTOGRAM_COUNTS_1M("DiskCache.TrimDelays", trim_delays_);
   trim_delays_ = 0;
   return true;
 }