cardano-testnet: remove possibility to programmatically pass a custom node configuration file (it was unused) #29063

Job	Run time
build (3.12, 9.6, macos-13, bash)	10m 59s
build (9.10, 3.12, ubuntu-latest, bash)	5m 24s
build (9.10, 3.12, windows-latest, C:/msys64/usr/bin/bash.exe -e {0})	10m 18s
build (9.6, 3.12, ubuntu-latest, bash)	6m 32s
build (9.6, 3.12, windows-latest, C:/msys64/usr/bin/bash.exe -e {0})	7m 0s
build-complete	1s
release	0s
	40m 14s

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