Remove FF_SHELL_EXECUTOR_USE_LEGACY_PROCESS_KILL feature flag

Overview

In #3376 (closed) we changed to we do termination for shell executor with !1770 (merged). We introduced a new feature flag FF_SHELL_EXECUTOR_USE_LEGACY_PROCESS_KILL which switches to the old way for terminating the process. Given that the new termination process works as expected for users we should remove this feature flag in %13.0