Revert "Use the abort strategy when tracing."

This reverts commit c8c964a.

The reason for this is that benchmark tests require the unwind strategy and
since benchmarks are mixed with normal tests in the same binary, it means that
all of our tests would have to use the unwind strategy.

See:
ykjit/yk#49 (comment)

src/librustc_session/config.rs

@@ -11,7 +11,7 @@ use crate::{early_error, early_warn, Session};
 use rustc_data_structures::fx::FxHashSet;
 use rustc_data_structures::impl_stable_hash_via_hash;
 
-use rustc_target::spec::{PanicStrategy, Target, TargetTriple};
+use rustc_target::spec::{Target, TargetTriple};
 
 use crate::parse::CrateConfig;
 use rustc_feature::UnstableFeatures;
@@ -1754,12 +1754,6 @@ pub fn build_session_options(matches: &getopts::Matches) -> Options {
         early_error(error_format, &format!("optimisation cannot be enabled with a tracer"));
     }
 
-    // If a tracer is enabled, we use only the abort panic strategy.
-    if cg.tracer != TracerMode::Off {
-        cg.panic = Some(PanicStrategy::Abort);
-        debugging_opts.panic_abort_tests = true;
-    }
-
     let cg = cg;
 
     // The `-g` and `-C debuginfo` flags specify the same setting, so we want to be able