Revert "Compute PPID at ninja time"

This reverts commit b1c17bb080ccf41b30909c0401df59e6f3a1ebb1.

It breaks make test-art-host when /bin/sh is not bash, as 
/bin/sh -c /bin/bash -c echo $PPID 
returns the pid of /bin/sh, not the pid of ninja.

Test: none
Change-Id: Ia4e5f76ea4c5f5b5c102ce54ae03c3fccb23a49f
2 files changed