Remove dead code that is never used in Windows Vista or later (part 2)

This is a follow up CL for r523 (Mozc 2.16.2040.102).

This CL is just a removal of dead code and redundant code.  Hence no behavior change is intended.

BUG=none
TEST=none

git-svn-id: https://mozc.googlecode.com/svn/trunk@537 a6090854-d499-a067-5803-1114d4e51264