std: Reduce checks for feature = "backtrace"

This is a stylistic change to libstd to reduce the number of checks of
`feature = "backtrace"` now that we unconditionally depend on the
`backtrace` crate and rely on it having an empty implementation.
otherwise.

src/libstd/thread/mod.rs

@@ -465,12 +465,9 @@ impl Builder {
             }
 
             thread_info::set(imp::guard::current(), their_thread);
-            #[cfg(feature = "backtrace")]
             let try_result = panic::catch_unwind(panic::AssertUnwindSafe(|| {
                 crate::sys_common::backtrace::__rust_begin_short_backtrace(f)
             }));
-            #[cfg(not(feature = "backtrace"))]
-            let try_result = panic::catch_unwind(panic::AssertUnwindSafe(f));
             *their_packet.get() = Some(try_result);
         };