Revert "build: compile_et rules for parallel make"

This reverts commit 6c3adb6db781ef4b15d9336a63b40d3a79b11264.
While gerrit 10310 may be correct, it at least triggers some
other problem, causing significantly higher failure rates for
parallel builds. This is especially true in combination with
gerrit 10337, where there's at least one known case of 100%
failure rate. A different solution, modifying compile_et to
allow it to emit a single file per invocation, is being worked
on. For the time being, revert 10310 on the 1.6 branch to get
parallel build stability back to the previous level.

Change-Id: I372d64868f7f7d0e7185f575212f16e453299b1c
Reviewed-on: http://gerrit.openafs.org/10349
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>
Reviewed-by: Marc Dionne <marc.c.dionne@gmail.com>
Reviewed-by: Andrew Deason <adeason@sinenomine.net>
Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>