selftests: net: move EXTRA_CLEAN of libynl.a into ynl.mk

Commit 1fd9e4f2 ("selftests: make kselftest-clean remove libynl outputs")
added EXTRA_CLEAN of YNL generated files to ynl.mk. We already had
a EXTRA_CLEAN in the file including the snippet. Consolidate them.

Acked-by: Stanislav Fomichev <sdf@fomichev.me>
Link: https://patch.msgid.link/20241011230311.2529760-2-kuba@kernel.org


Signed-off-by: Jakub Kicinski <kuba@kernel.org>