git p4 test: simplify timeout handling

git p4 test: simplify timeout handling

'lib-git-p4.sh' uses timeouts in a watchdog process to kill a
potentially stuck 'p4d' process and for certain cleanup operation
between tests.  It does so by first computing when the timeout should
expire, and then repeatedly asking for the current time in seconds
until it exceeds the expiration time, and for portability reasons it
uses a one-liner Python script to ask for the current time.

Replace these timeouts with downcounters, which, though not
necessarily shorter, are much simpler, at least in the sense that they
don't execute the Python interpreter every second.

After this change the helper function with that Python one-liner has
no callers left, remove it.

Signed-off-by: SZEDER Gábor <szeder.dev@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>