Remove the GenerateVariableType from the CheckerGenerator interface.

GenerateVariableType was an unused function outside of some internal pieces.
Thus remove it from the CheckerGenerator interface, as it doesn't need to be
exposed.  Update a few internal uses of it to use an explicit instance of a
CheckerGenerator for when it was used (inside of tests).

git-svn-id: https://groke.mcmaster.ca/svn/grad/colin/branches/TableTool_javization@10654 57e6efec-57d4-0310-aeb1-a6c144bb1a8b