crash_sender: delete redundant logic for parallel runs

A recent commit added global locking support via `flock` (see CL:65556).
With that in place, we no longer need this ad-hoc pid checking.  Advantage
is that the flock is race free (both with file reads/writes and pid #s).

BUG=chromium:199491
TEST=`cbuildbot x86-generic-full` works
TEST=`crash_sender` on board works
CQ-DEPEND=CL:201495

Change-Id: I00d48544856cc5fad60c6129f5c193abdde45f02
Reviewed-on: https://chromium-review.googlesource.com/169486


Reviewed-by: Rohit Makasana <rohitbm@chromium.org>
Commit-Queue: Mike Frysinger <vapier@chromium.org>
Tested-by: Mike Frysinger <vapier@chromium.org>