writerfilter: Kill CreateElement and CreateElementMap.

The last bogus usage unordered_map is now gone.  Saves another 300k (stripped
dbgutil build).

Change-Id: I7be0c495d119e4b117eacc6eb5a5f19433582f73
writerfilter: Move includes accordingly.

Change-Id: I35c27dfa459fff3a2ee271333980c789d01fac56