lib,src: remove post-gc event infrastructure

Remove the 'gc' event from the v8 module and remove the supporting
infrastructure from src/.  It gets the axe because:

1. There are currently no users.  It was originally conceived as
   an upstreamed subset of StrongLoop's strong-agent GC metrics,
   but the strong-agent code base has evolved considerably since
   that time and has no use anymore for what is in core.

2. The implementation is not quite sound.  It calls into JS land
   from inside the GC epilog and that is unsafe.  We could fix
   that by delaying the callback until a safe time but because
   there are no users anyway, removing it is all around easier.

R=@trevnorris