Deprecate host-side C++ test in LatinIME

Host-side C++ test in LatinIME is no longer actively used and is
actually not compatible with Android Studio build.

Let's just remove this for now.

Bug: 13754552
Bug: 110741422
Test: compile
Change-Id: I3e81285fe559022e1a296051036e74d2f2afbf9c