CORES environment variable set by many launchers seems to be unused

[benclifford]

Describe the bug

Many launchers in parsl/launchers/launchers.py set a CORES environment variable. No other part of Parsl seems to use that. The value seems maybe dubiously set too: it sets the number of CORES on the host running the submit script, which is not necessarily a worker node, and is not necessarily any indication of how many cores can be used by any particular process? (particularly, the process_worker_pool has other mechanisms for configuring how many cores to use)

Maybe this is cruft that can be removed?