Don't reject *multiple* relaxed bounds, reject *duplicate* ones.

Having multiple relaxed bounds like `?Sized + ?Iterator` is actually *fine*.
We actually want to reject *duplicate* relaxed bounds like `?Sized + ?Sized`
because these most certainly represent a user error.

Note that this doesn't mean that we accept more code because a bound like
`?Iterator` is still invalid as it's not relaxing a *default* trait and
the only way to define / use more default bounds is under the experimental
and internal feature `more_maybe_bounds` plus `lang_items` plus unstable
flag `-Zexperimental-default-bounds` (historical context: for the longest
time, bounds like `?Iterator` were actually allowed and lead to a hard
warning).

Ultimately, this simply *reframes* the diagnostic. The scope of
`more_maybe_bounds` / `-Zexperimental-default-bounds` remains unchanged
as well.

compiler/rustc_error_codes/src/error_codes/E0203.md

@@ -1,15 +1,15 @@
-Having multiple relaxed default bounds is unsupported.
+Having duplicate relaxed default bounds is unsupported.
 
 Erroneous code example:
 
 ```compile_fail,E0203
-struct Bad<T: ?Sized + ?Send>{
-    inner: T
+struct Bad<T: ?Sized + ?Sized>{
+    inner: T,
 }
 ```
 
-Here the type `T` cannot have a relaxed bound for multiple default traits
-(`Sized` and `Send`). This can be fixed by only using one relaxed bound.
+Here the type parameter `T` cannot have duplicate relaxed bounds for default
+trait `Sized`. This can be fixed by only using one relaxed bound:
 
 ```
 struct Good<T: ?Sized>{