Revert "Clarify how SLURM_SUBMIT_DIR is set in salloc/sbatch/srun man pages."

This reverts commit 69d5d94b.

submit_dir != working dir

In 7092, the customer was seeing the interactive srun set
SLURM_SUBMIT_DIR to the cwd in the interactive srun after -D had been
applied -- which is wrong behavior. This will be fixed in 21.08.

Bug 11407