Removed obsolete APIs form v2.

Removed MSGPACK_DISABLE_LEGACY_CONVERT from v2. Those APIs are removed
from v2.

example/cpp03/CMakeLists.txt

@@ -5,12 +5,17 @@ LIST (APPEND exec_PROGRAMS
     custom.cpp
     enum.cpp
     map_based_versionup.cpp
-    protocol.cpp
     protocol_new.cpp
     reuse_zone.cpp
     simple.cpp
 )
 
+IF (MSGPACK_DEFAULT_API_VERSION EQUAL 1)
+    LIST (APPEND exec_PROGRAMS
+        protocol.cpp
+    )
+ENDIF ()
+
 IF (NOT MSVC)
     LIST (APPEND with_pthread_PROGRAMS
         stream.cpp