MSE: Remove incorrect obsolete code for seek to current time

Removes the (broken) condition whereby a seek on an open MediaSource
to current time would have not flushed the source.
The requirement for flushing as part of MSE seek is obsolete,
so this change removes this block of code.
Found while investigating flaky-failing mediasource-duration.html

R=acolwell@chromium.org
TEST=no media or http/tests/media regression locally on Linux
BUG=284782

Review URL: https://codereview.chromium.org/293253004

git-svn-id: svn://svn.chromium.org/blink/trunk@174902 bbb929c8-8fbe-4397-9dbb-9b2b20218538