Delete functions instead of using CVC4_UNDEFINED (#1794)

Delete functions instead of using CVC4_UNDEFINED (#1794)

C++11 supports explicitly deleting functions that should not be used
(explictly or implictly), e.g. copy or assignment constructors. We were
previously using the CVC4_UNDEFINED macro that used a compiler-specific
attribute. The C++11 feature should be more portable.