builder: Remove pids from list of running jobs when job has finished.
Revert "A new try to get the parallelism straight."

This reverts commit 369ac3b365091ba1a26fb266a94d97adb3719cbc
and obviously this is not a solution either.
build parallelism: New way to figure that out.

The system will now use either the old formula,
CPU cores * 2 + 1 or the amount of memory in megabytes devided
by 128 (i.e. each compiler process can have 128M RAM) which ever
is less.