fix: remove freq2wavelength that is already defined #277

Job	Run time
Tests on paywalled platforms	0s
Tests on other platforms (windows-2019, 3.10)	3m 21s
Tox test (py38)	1m 54s
Tests on other platforms (windows-2022, 3.11)	2m 46s
Tox test (py39)	1m 53s
Tests on other platforms (windows-2022, 3.12)	2m 31s
Tests on other platforms (macos-12, 3.11)	2m 19s
Tox test (py310)	1m 46s
Tests on other platforms (macos-13, 3.12)	1m 35s
Tox test (py311)	1m 50s
Tox test (py312-cover)	2m 53s
Tox test (docs, graphviz)	1m 57s
Docker image	0s
PyPI packaging	0s
	24m 45s

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