build: Remove obsolete karma:bg task

Follows-up 829f4c3. Since we now run browsers in the 'karma:other'
task already, we already spawn the browser for each run, so
'karma:bg' would only run them a second time for no reason. This
hack is no longer needed.

Change-Id: I4d10e8d46a2feb61df61282e04b42d51ee23cfb1