[OMPT] Remove test output from source tree

[Hahnfeld]

Summary

%s refers to the test file in the source tree. This was accidentally added in
r351197 / 2b46d30 ("[OMPT] Second chunk of final OMPT 5.0 interface updates").

[protze.joachim]

LGTM, thanks for catching this!
The output is written to the build dir anyways:

runtime/test/lit.cfg:127

config.substitutions.append(("FileCheck", "tee %%t.out | %s" % config.test_filecheck))

openmp/runtime/test/ompt/tasks/explicit_task.c

-// RUN: %libomp-compile-and-run | %sort-threads | tee %s.out | FileCheck %s
+// RUN: %libomp-compile-and-run | %sort-threads | FileCheck %s
 // REQUIRES: ompt
 // UNSUPPORTED: gcc-4, gcc-5, gcc-6, gcc-7
 #define TEST_NEED_PRINT_FRAME_FROM_OUTLINED_FN
 #include "callback.h"
 #include <omp.h> 
 
 int main()
 {