flambda-backend: Fix backtrace_c_exn.opt.reference

Author: mshinwell

testsuite/tests/backtrace/backtrace_c_exn.opt.reference

@@ -1,5 +1,4 @@
 Failure("exn")
-Raised at Stdlib.failwith in file "stdlib.ml" (inlined), line 34, characters 17-33
-Called from Backtrace_c_exn.raise_exn in file "backtrace_c_exn.ml", line 14, characters 19-33
+Raised at Stdlib.failwith in file "stdlib.ml", line 34, characters 17-33
 Called from Backtrace_c_exn in file "backtrace_c_exn.ml", line 20, characters 4-20
 Called from Backtrace_c_exn in file "backtrace_c_exn.ml", line 20, characters 4-20