selftests: net: remove redundant rp_filter configuration

selftests: net: remove redundant rp_filter configuration

The following tests use setup_ns to create a network namespace, which
will disables rp_filter immediately after namespace creation. Therefore,
it is no longer necessary to disable rp_filter again within these individual
tests.

Signed-off-by: Hangbin Liu <liuhangbin@gmail.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Link: https://patch.msgid.link/20250508081910.84216-3-liuhangbin@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>