Revert "Display teardown: Fix race condition in flushing logic"

This reverts commit e051ba8919b4d75ca3bacf58087d525e84354e08.

fadfe6e added a flush during pipeline teardown, to avoid what we call
"long teardown" with non-live sources like filesrc -- the pipeline
doesn't actually get the EOS within 10 seconds so stbt times out and
exits forcefully.

Unfortunately this caused a live v4l source (real video-capture
hardware) to always suffer from long teardown.

e051ba8 changed the order of sending eos vs. flush_stop, for an
unrelated reason (to fix an intermittent long teardown found with a
non-live source in our self-tests). e051ba8 also improved the long
teardown with our live source, from 100% of test runs to ~2% of test
runs.

I am reverting both fadfe6e and e051ba8 -- better teardown with non-live
sources isn't worth it if it causes worse teardown with live sources.
Non-live sources are only useful for our self-tests.
Revert "Display: Fix ungraceful teardowns (with delays) with non-live sources"

This reverts commit fadfe6e39c40bd8d70845c218341957526917a4c.

fadfe6e added a flush during pipeline teardown, to avoid what we call
"long teardown" with non-live sources like filesrc -- the pipeline
doesn't actually get the EOS within 10 seconds so stbt times out and
exits forcefully.

Unfortunately this caused a live v4l source (real video-capture
hardware) to always suffer from long teardown.

e051ba8 changed the order of sending eos vs. flush_stop, for an
unrelated reason (to fix an intermittent long teardown found with a
non-live source in our self-tests). e051ba8 also improved the long
teardown with our live source, from 100% of test runs to ~2% of test
runs.

I am reverting both fadfe6e and e051ba8 -- better teardown with non-live
sources isn't worth it if it causes worse teardown with live sources.
Non-live sources are only useful for our self-tests.
Merge pull request #177 from drothlis/fix-intermittent-long-teardown

Fix intermittent long teardown