liblog: don't set transports to nullptr when they close

The old code did not free the transports, just unlink their nodes from
the linked list, so there was no race condition between close() and
other threads writing to the logs.  By settings these to nullptr, I
introduced a race condition, but setting them to nullptr isn't
necessary, so let's simply not do that.

Test: liblog-unit-tests
Change-Id: Iec0b680addd13b5d30bd91ccef5bdeab6bf797b0