Removing MooseVariableBase::index() (closes #3094)

It is not needed, since it returns the variable number which is the
task done by number().
Merge pull request #3098 from andrsd/3094

Removing MooseVariableBase::index() (closes #3094)
Merge commit '3410e091c6ce9a7bc5e42fcab649d68355ed8d1f'