The "cylc submit" command should not write a suite env file.

This became a bug in cylc-5.4.5 when we made "cylc submit" use the suite
run directory, to allow it to work with Rose suites (which may install
files required by tasks into the proper suite run directory).
Added to cylc submit command help.
Merge pull request #894 from hjoliver/fix-submit-command-bug

The "cylc submit" command should not write a suite env file.