writerfilter: Kill ListValueMap(s) & their construction.

ListValueMaps were completely useless; it is always just few values that used
to be in the unordered_map, so doing just a normal comparison couldn't be too
much worse than all the hash construction etc.

This saves >300k of the libwriterfilter.so size (striped).

If this proves to be less performing than what we used to have (which I
doubt), now when we generate the code using Python, it would be a piece of
cake to construct some deterministic finite automaton in each switch's case to
accept the value names, and return the values.

Change-Id: I95d0470b73ade1fbf5c9de2299c4adcc59286bc3