child_process: remove unreachable execSync() code

Code coverage showed that the execSync() variable inheritStderr
was never set to the default value of true. This is because
the default case is hit whenever normalizeExecArgs() returns an
object without an 'options' property. However, this can never
be the case because normalizeExecArgs() unconditionally creates
the options object. This commit removes the unreachable code.

PR-URL: https://github.com/nodejs/node/pull/9209


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Sakthipriyan Vairamani <thechargingvolcano@gmail.com>