Revert "Normalize function type during validation"

This reverts commit d486bfcbff.

src/test/ui/issues/issue-50865-private-impl-trait/auxiliary/lib.rs

@@ -1,3 +1,7 @@
+// revisions: default miropt
+//[miropt]compile-flags: -Z mir-opt-level=2
+// ~^ This flag is for #77668, it used to be ICE.
+
 #![crate_type = "lib"]
 
 pub fn bar<P>( // Error won't happen if "bar" is not generic