integration: remove exit node via grant tests

Remove TestGrantViaExitNodeSteering and TestGrantViaMixedSteering.
Exit node traffic forwarding through via grants cannot be validated
with curl/traceroute in Docker containers because Tailscale exit nodes
strip locally-connected subnets from their forwarding filter.

The correctness of via exit steering is validated by:
- Golden MapResponse comparison (TestViaGrantMapCompat with GRANT-V31
  and GRANT-V36) comparing full netmap output against Tailscale SaaS
- Filter rule compatibility (TestGrantsCompat with GRANT-V14 through
  GRANT-V36) comparing per-node PacketFilter rules against Tailscale SaaS
- TestGrantViaSubnetSteering (kept) validates via subnet steering with
  actual curl/traceroute through Docker, which works for subnet routes

Updates #2180

.github/workflows/test-integration.yaml

@@ -249,8 +249,6 @@ jobs:
           - TestAutoApproveMultiNetwork/webauth-group.*
           - TestSubnetRouteACLFiltering
           - TestGrantViaSubnetSteering
-          - TestGrantViaExitNodeSteering
-          - TestGrantViaMixedSteering
           - TestHeadscale
           - TestTailscaleNodesJoiningHeadcale
           - TestSSHOneUserToAll