Revert "[llvm-exegesis] Introduce Subprocess Executor Mode"

This reverts commit 0d4ef4ff.

This was causing build failures on certain platforms when built with
-Werror due to unused variable warnings in addition to causing build
failures on Linux systems with older kernel versions as kernels prior to
v5.15 don't support sys_pidfd_getpid. Reverting as I need to setup a
system to properly test the rest of the patches in this series.

Also reverts 8c6668fa which fixed the
first issue so that the patch can actually be reverted.