remove nad subdirectory from consideration in PROJ build

This breaks cross-compiled Bazel builds due to
OSGeo#1235. We appear to never need
`nad2bin`, so we can safely just exclude this entire directory.

CMakeLists.txt

@@ -120,7 +120,6 @@ set(INCLUDEDIR "${DEFAULT_INCLUDEDIR}" CACHE PATH "The directory to install incl
 include_directories(${PROJ4_SOURCE_DIR}/src)
 
 message(STATUS "")
-add_subdirectory(nad)
 add_subdirectory(src)
 add_subdirectory(man)
 add_subdirectory(cmake)