Revert "scripts/Makefile.headersinst: make scratch file parallel safe"

This reverts commit 69005c76. This commit
was a bugfix on top of commit 71f613a0.
As this commit is also being reverted, we no longer need this bugfix.

Change-Id: I7f9a29d78e1e0ddf25f84c993365fcabdc0880e7
Signed-off-by: Ian Maund <imaund@codeaurora.org>