Remove empty test

src/test/mir-opt/dataflow-const-prop/cell.main.DataflowConstProp.diff

@@ -1,12 +0,0 @@
-- // MIR for `main` before DataflowConstProp
-+ // MIR for `main` after DataflowConstProp
-  
-  fn main() -> () {
-      let mut _0: ();                      // return place in scope 0 at $DIR/cell.rs:+0:11: +0:11
-  
-      bb0: {
-          _0 = const ();                   // scope 0 at $DIR/cell.rs:+0:11: +2:2
-          return;                          // scope 0 at $DIR/cell.rs:+2:2: +2:2
-      }
-  }
-  

src/test/mir-opt/dataflow-const-prop/cell.rs

@@ -1,6 +0,0 @@
-// unit-test: DataflowConstProp
-
-// EMIT_MIR cell.main.DataflowConstProp.diff
-fn main() {
-    // FIXME: Is it possible to build something with UnsafeCell?
-}