back compat: remove cli_format

[oliver-sanders]

Address this BACK COMPAT notice:

cylc-flow/cylc/flow/util.py

Lines 197 to 203 in 1e45d07

	BACK_COMPAT: cli_format
	From:
	Python 3.7
	To:
	Python 3.8
	Remedy:
	Can replace with shlex.join