Revert "[benchmark] Re-enable building benchmark tests"

This reverts commit c9eda518d5f926982827154f240134f40f0b3b89.

With this change the test suite is failing to get past cmake
on all of our (Linaro's) Arm bots:
https://lab.llvm.org/buildbot/#/builders/178/builds/151/steps/19/logs/stdio

CMake Error at cmake/modules/TestSuite.cmake:99 (add_dependencies):
  Cannot add target-level dependencies to non-existent target
  "donotoptimize_assembly_test".
1 file changed