logd: don't use SIGHUP to reinitialize

It doesn't look like there are any users, since there is a much better
mechanism, `logd --reinit` that exists for this behavior.  The
settings app and init.rc use that mechanism and they are the two major
clients that force logd to reinitialize.

This saves us from creating a thread and marginally cleans up
main.cpp.

Test: log reinitialization works
Change-Id: Icdb56b6f59adbba82812231a4f3a6ffd1a7955fc