Remove compiletest comments from tests

src/test/ui/block-expression-remove-semicolon.stderr

@@ -3,8 +3,8 @@ error[E0308]: mismatched types
    |
 LL |       let x: i32 = {
    |  __________________^
-LL | |         //~^ ERROR mismatched types
-LL | |         foo(); //~ HELP consider removing this semicolon
+LL | |
+LL | |         foo();
    | |              - help: consider removing this semicolon
 LL | |     };
    | |_____^ expected i32, found ()

src/test/ui/issues/issue-46101.stderr

 error[E0433]: failed to resolve: partially resolved path in a derive macro
   --> $DIR/issue-46101.rs:3:10
    |
-LL | #[derive(Foo::Anything)] //~ ERROR failed to resolve: partially resolved path in a derive macro
+LL | #[derive(Foo::Anything)]
    |          ^^^^^^^^^^^^^ partially resolved path in a derive macro
 
 error[E0601]: `main` function not found in crate `issue_46101`

src/test/ui/issues/issue-55731.stderr

 error: implementation of `DistributedIteratorMulti` is not general enough
   --> $DIR/issue-55731.rs:48:5
    |
-LL |     multi(Map { //~ ERROR implementation of `DistributedIteratorMulti` is not general enough
+LL |     multi(Map {
    |     ^^^^^
    |
    = note: `DistributedIteratorMulti<&'0 ()>` would have to be implemented for the type `Cloned<&()>`, for any lifetime `'0`