tests: remove more old async tests

The rejection of #6025 indicates that if stackless coroutines return to
Zig, they will look quite different; see #23446 for the working draft
proposal for their return (though it will definitely be tweaked before
being accepted). Some of this test coverage was deleted in 40d11cc, but
because stackless coroutines will take on a new form if re-introduced, I
anticipate that essentially *none* of this coverage will be relevant. Of
course, if it for some reason is, we can always grab it from the Git
history.

Author: mlugg

test/behavior/type_info.zig

@@ -539,20 +539,6 @@ fn add(a: i32, b: i32) i32 {
     return a + b;
 }
 
-test "type info for async frames" {
-    if (true) {
-        // https://github.com/ziglang/zig/issues/6025
-        return error.SkipZigTest;
-    }
-
-    switch (@typeInfo(@Frame(add))) {
-        .frame => |frame| {
-            try expect(@as(@TypeOf(add), @ptrCast(frame.function)) == add);
-        },
-        else => unreachable,
-    }
-}
-
 test "Declarations are returned in declaration order" {
     if (builtin.zig_backend == .stage2_sparc64) return error.SkipZigTest; // TODO
     if (builtin.zig_backend == .stage2_spirv) return error.SkipZigTest;