have a single union ubuf definition in lmptype.h and remove others

src/compute.h

@@ -161,17 +161,6 @@ class Compute : protected Pointers {
     return j >> SBBITS & 3;
   }
 
-  // union data struct for packing 32-bit and 64-bit ints into double bufs
-  // see atom_vec.h for documentation
-
-  union ubuf {
-    double d;
-    int64_t i;
-    ubuf(double arg) : d(arg) {}
-    ubuf(int64_t arg) : i(arg) {}
-    ubuf(int arg) : i(arg) {}
-  };
-
   // private methods
 
   void adjust_dof_fix();