liblog: remove unused log_time functions, inline the others

One of the reasons that logcat and logd statically include liblog is
to access the symbols in log_time.cpp, which we do not expose
otherwise.  Except for strptime(), which will be handled in a separate
CL, these symbols are either small enough to inline in the header or
unused and can be removed.

Test: logging unit tests
Change-Id: I1f8cfbb779aef79fc7d5b6d0050438fe5f0e0e2c