Remove unused `invalid_insert_overwrite_delta_msg` variable #2459

Job	Run time
build packages	18s
code-quality	1m 48s
unit test / python 3.8	1m 4s
unit test / python 3.9	1m 7s
unit test / python 3.10	1m 11s
unit test / python 3.11	1m 1s
verify packages / python ${{ matrix.python-version }} / ${{ matrix.os }}	0s
	6m 29s

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