Drop fn destroy support (#6136)

`fn destroy` is being removed per decision on #6124. It seems like the
relevant decision will result in no more keyword-based function names,
so this is removing all related support.

toolchain/lower/testdata/function/definition/destroy.carbon

@@ -16,7 +16,7 @@
 library "[[@TEST_NAME]]";
 
 class HasDestroy {
-  // TODO: Use `fn destroy` instead once lowering supports it.
+  // TODO: Change for `Destruction`+`Destroy`.
   impl as Core.Destroy {
     fn Op[self: Self]();
   }