Remove unused old code from VDL/VOM.

This is stuff leftover from the transition.

This CL tries to keep the diffs reasonable, so I haven't merged
xdecoder.go and decoder.go yet.  That'll come in a separate CL.

MultiPart: 1/2
Change-Id: Ie7c01bb854f073faeffe82337d5a6f0247a1d2e0
26 files changed