parallel_51 increments a shared variable array and then decrements it
This change deletes that. It leaves the comments in place. Change introduced
in D95976 which notes the implementation is a stopgap. Even if this is not code
which is expected to work, I'd like to drop the accesses to parallel level as
that simplifies reasoning about the other uses of it.
Opt is not presently able to detect that the increment then decrement have no
net effect so this change decreases the size of the parallel_51 function.