remove use of feature(trait_upcasting) from core tests

Author: WaffleLapkin

library/core/tests/lib.rs

@@ -1,4 +1,5 @@
 // tidy-alphabetical-start
+#![cfg_attr(bootstrap, feature(trait_upcasting))]
 #![cfg_attr(target_has_atomic = "128", feature(integer_atomics))]
 #![cfg_attr(test, feature(cfg_match))]
 #![feature(alloc_layout_extra)]
@@ -81,7 +82,6 @@
 #![feature(strict_provenance_atomic_ptr)]
 #![feature(strict_provenance_lints)]
 #![feature(test)]
-#![feature(trait_upcasting)]
 #![feature(trusted_len)]
 #![feature(trusted_random_access)]
 #![feature(try_blocks)]