Refactor: fencer: Minor GSource improvements

• clear_remote_op_timers() is idempotent and protects against double GSource remove. So for safety we can call it first in finalize_op() to guarantee that the timer sources are freed.
• There's no need to zero out op->op_timer_total within finalize_timed_out_op(), since it calls finalize_op().
• There's no need to remove op->op_timer_total and set it to zero if we're about to call finalize_timed_out_op(), for the same reason.
• Return G_SOURCE_REMOVE instead of FALSE. G_SOURCE_REMOVE is defined as FALSE but is preferred for clarity.

We continue to explicitly set op->op_timer_one to 0 in
remote_op_watchdog_done(). The call to finalize_op() makes this
redundant, but it's good to be consistent in removing the appropriate
timer source at the beginning of each GSourceFunc (timer callback).

Signed-off-by: Reid Wahl <nrwahl@protonmail.com>