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

integration/helpers.go

@@ -1007,13 +1007,6 @@ func tagApprover(name string) policyv2.AutoApprover {
 	return new(policyv2.Tag(name))
 }
 
-// autogroupp returns a pointer to an AutoGroup as an Alias for policy v2 configurations.
-// Used in grant rules to reference autogroups like autogroup:self and autogroup:internet.
-func autogroupp(name string) policyv2.Alias {
-	ag := policyv2.AutoGroup(name)
-	return &ag
-}
-
 // oidcMockUser creates a MockUser for OIDC authentication testing.
 // Generates consistent test user data with configurable email verification status
 // for validating OIDC integration flows in headscale authentication tests.