gio: Drop redundant g_source_is_destroyed() calls

These calls cause race warnings from tsan, but are not a thread safety
problem, because we can only ever observe single bit changes: all
modifications to the GSource.flags field are done with a lock held; all
reads are of independent fields, so no intermediate state can ever be
observed. This assumes that a non-atomic read will consistently give us
an old value or a new value.

In any case, these g_source_is_destroyed() calls can happen from any
thread, and the state could be changed from another thread immediately
after the call returns; so the checks are pointless. In addition,
calling g_source_set_ready_time() or g_source_destroy() on a destroyed
source is not a problem.

https://bugzilla.gnome.org/show_bug.cgi?id=778049