Revert "util/timer: avoid deadlock when shutting down"

Revert "util/timer: avoid deadlock when shutting down"

This reverts commit bc02be4508d8753d1f6071b77d10f4661587df6f.

Now we catch attempts to clock_step to the next timer when none are
enabled we can revert the previous attempt to prevent deadlock. As
long as a new target time is given we will move time forward even if
no timers will fire. This is desirable for tests which are checking
that nothing changes when things are disabled.

Previously most tests got away with it because --enable-slirp always
has a timer running while the test is active.

Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Message-Id: <20250207153112.3939799-8-alex.bennee@linaro.org>