replace feature expression (cfg_panic) in lib and remove expression from tests

Rebase commit

library/core/tests/lib.rs

@@ -6,7 +6,7 @@
 #![feature(bool_to_option)]
 #![feature(box_syntax)]
 #![feature(cell_update)]
-#![feature(cfg_panic)]
+#![cfg_attr(bootstrap,feature(cfg_panic))]
 #![cfg_attr(bootstrap, feature(cfg_target_has_atomic))]
 #![feature(const_assume)]
 #![feature(const_black_box)]

src/test/ui/cfg/cfg-panic-abort.rs

@@ -1,7 +1,7 @@
 // build-pass
 // compile-flags: -C panic=abort
 // no-prefer-dynamic
-#![feature(cfg_panic)]
+
 
 #[cfg(panic = "unwind")]
 pub fn bad() -> i32 { }

src/test/ui/cfg/cfg-panic.rs

@@ -4,7 +4,7 @@
 // ignore-emscripten no panic_unwind implementation
 // ignore-wasm32     no panic_unwind implementation
 // ignore-wasm64     no panic_unwind implementation
-#![feature(cfg_panic)]
+
 
 #[cfg(panic = "abort")]
 pub fn bad() -> i32 { }

src/test/ui/fmt/format-args-capture.rs

@@ -1,5 +1,4 @@
 // run-pass
-#![feature(cfg_panic)]
 
 fn main() {
     named_argument_takes_precedence_to_captured();

src/test/ui/issues/issue-68696-catch-during-unwind.rs

@@ -4,7 +4,6 @@
 // entering the catch_unwind.
 //
 // run-pass
-#![feature(cfg_panic)]
 
 use std::panic::catch_unwind;