Drop index from mesh element group constructor #2250

Job	Run time
Pytest Conda Py3	11m 47s
Mypy	3m 38s
Documentation	3m 55s
Tests for downstream project grudge	9m 41s
Pytest Firedrake	6m 23s
Examples Conda Py3	4m 51s
Tests for downstream project pytential	33m 0s
Examples Firedrake	3m 19s
Tests for downstream project mirgecom	3s
Tests for downstream project mirgecom_examples	5s
Flake8	17s
Pylint	3m 53s
	1h 20m 52s

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