Remove dupe bug detection from notify_impact

As DocImpact is now only triggered on patchset-merge, this code
is redundant.

In addition, I believe Launchpad search has started to return false
positive matches, meaning that this code is preventing some DocImpact
bugs from being created.

Change-Id: Id2e7155dff7b3562ba5697f3988682ef8e4acdcd
Merge "Remove dupe bug detection from notify_impact"