rb_cData: no longer exists

Commit 8918a9cf6c introduced macro
`#define rb_cData rb_cData()`.  This deleting `VALUE rb_cData;`
declaration was then macro-expanded into `VALUE rb_cData();`.  This
worked by accident because the expanded expression happen to be a K&R
style function declaration.

This is rather complicated and I guess unintended.  Just delete the line
to keep things simple straight forward.

object.c

@@ -49,7 +49,6 @@ VALUE rb_mKernel; /*!< Kernel module */
 VALUE rb_cObject; /*!< Object class */
 VALUE rb_cModule; /*!< Module class */
 VALUE rb_cClass; /*!< Class class */
-VALUE rb_cData; /*!< Data class */
 
 VALUE rb_cNilClass; /*!< NilClass class */
 VALUE rb_cTrueClass; /*!< TrueClass class */