selftests/nolibc: drop implicit defconfig executions

selftests/nolibc: drop implicit defconfig executions

Commit d7d271ec30dd ("selftests/nolibc: execute defconfig before other targets")
accidentally introduced implicit executions of the defconfig target.
These executions were unintentional and come from a misunderstanding of
ordering dependencies.

Drop the dependencies again.

Reported-by: Mark Brown <broonie@kernel.org>
Closes: https://lore.kernel.org/all/3d5128b9-b4b6-4a8e-94ce-ea5ff4ea655b@sirena.org.uk/
Acked-by: Willy Tarreau <w@1wt.eu>
Link: https://lore.kernel.org/r/20250620-nolibc-selftests-v1-1-f6b2ce7c5071@weissschuh.net
Signed-off-by: Thomas Weißschuh <linux@weissschuh.net>