Benchtests: Remove simple_strcspn/strpbrk/strsep

Remove simple_strcspn/strpbrk/strsep which are significantly slower than the
generic implementations.  Also remove oldstrsep and oldstrtok since they are
practically identical to the generic implementation.  Adjust iteration count
to reduce benchmark time.

Reviewed-by: Adhemerval Zanella <adhemerval.zanella@linaro.org>