libgfx: Remove SafeLog

SafeLog is being removed from libgfx (which is being dismantled) and
split into two components:

    1) A Loggable mixin which will live in liblog and provide the basic
       class logging functionality SafeLog provided
    2) A FmtLogger plugin for the Loggable mixin which will live in
       external/fmtlib and allow the use of fmtlib for logging

Test: Manual testing of both the default and FmtLogger paths
Change-Id: I562f4816bf84c9cb4e0f7b2593429f72a0907288