chore: remove feature gate CombinedRoutes

[programmer04]

What this PR does / why we need it:

CombinedRoutes graduated to be always enabled and not configurable, remove the feature gate, all code related to handling old behavior, and adjust tests.

Which issue this PR fixes:

Closes #3131

Special notes for your reviewer:

Golden tests **/combined-routes-off_golden.yaml have been removed, because now it can't be disabled.
As I checked no changes are required in Helm charts.
Please point me in case of miss a place where I should make an update.

PR Readiness Checklist:

Complete these before marking the PR as ready to review:

• the CHANGELOG.md release notes have been updated to reflect any significant (and particularly user-facing) changes introduced by this PR

[team-k8s-bot]

E2E (targeted) tests with KIND-based clusters were started at https://github.com/Kong/kubernetes-ingress-controller/actions/runs/6337549972

[codecov]

Codecov Report

Attention: 7 lines in your changes are missing coverage. Please review.

Comparison is base (95b0447) 67.9% compared to head (f790020) 67.7%.
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##            main   #4749     +/-   ##
=======================================
- Coverage   67.9%   67.7%   -0.2%     
=======================================
  Files        165     165             
  Lines      19291   19094    -197     
=======================================
- Hits       13102   12931    -171     
+ Misses      5420    5400     -20     
+ Partials     769     763      -6     

Files	Coverage Δ
internal/dataplane/kongstate/kongstate.go	74.9% <100.0%> (+0.8%)	⬆️
internal/dataplane/parser/parser.go	83.3% <100.0%> (-1.1%)	⬇️
internal/dataplane/parser/translate_ingress.go	96.4% <ø> (+0.4%)	⬆️
internal/manager/featuregates/feature_gates.go	100.0% <100.0%> (ø)
internal/dataplane/parser/translate_httproute.go	73.5% <84.2%> (-4.1%)	⬇️
internal/admission/validation/ingress/ingress.go	0.0% <0.0%> (ø)

... and 6 files with indirect coverage changes

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[czeslavo]

Huh, it's good to see all this code going away :)