test: remove the just added, but now unrealistic, shake128 OAEP tests #26060

Job	Run time
basic_clang	5m 43s
check_docs	1m 3s
check-ansi	2m 33s
check_update	1m 42s
minimal	2m 33s
self-hosted (freebsd-13.2)	14m 11s
address_ub_sanitizer	10m 44s
basic_gcc	5m 48s
self-hosted (ubuntu-arm64-22.04)	9m 49s
enable_non-default_options	4m 47s
full_featured	6m 0s
fuzz_tests	2m 58s
memory_sanitizer	21m 49s
no-deprecated	5m 23s
no-legacy	5m 35s
non-caching	16m 4s
no-shared-macos (macos-13)	7m 45s
no-shared-ubuntu	4m 41s
no-shared-macos (macos-14)	3m 49s
threads_sanitizer	5m 21s
external-test-pyca (1.51.0, 3.9)	6m 51s
external-tests	11m 19s
external-test-cf-quiche	3m 32s
legacy	4m 44s
out-of-readonly-source-and-install-macos (macos-13)	11m 34s
out-of-readonly-source-and-install-macos (macos-14)	5m 46s
out-of-readonly-source-and-install-ubuntu	7m 39s
	3h 9m 43s

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