Revert "[NFC][LSAN] Limit the number of concurrent threads is the test"

This reverts commit 2a73b7bd.

This appears to be causing the following failures on GreenDragon:
  LeakSanitizer-AddressSanitizer-x86_64 :: TestCases/many_threads_detach.cpp
  LeakSanitizer-Standalone-x86_64 :: TestCases/many_threads_detach.cpp

First failure:
    http://green.lab.llvm.org/green/job/clang-stage1-RA/20754/

Still failing in latest build:
    http://green.lab.llvm.org/green/job/clang-stage1-RA/20928/