Remove old check for duplicate subdirectories

Remove old check for duplicate subdirectories

In cmMakefile::AddSubDirectory we were checking for addition of the same
source directory multiple times.  However, the check code was incorrect
because it compared pointers instetad of pointed-to strings.  Since the
check was written, a better check was added right after it to enforce
unique binary directories (in which case duplicate sources are fine).
This commit simply removes the old-style check code.