Revert "Work around HHVM bug with stdin/stdout in proc_open()"

This was fixed in HHVM (https://github.com/facebook/hhvm/issues/1247).

This reverts commit 52e546640c5c0c0e817ae1fc5451a934b2c34de9.

bug: 56597
Change-Id: I9c97ba5645d794ff562347979e3f2027eb8b4534