refactor: delete custom entities

[pmalek]

What this PR does / why we need it:

It seems that custom entities where never used (at least in a handful of recent versions).

The notion of them was "somehow" introduced in #1339 and #2296 made sendconfig.PerformUpdate() always pass nil in place for "custom entities".

Since it's always nil we may as well just delete it altogether.

Special notes for your reviewer:

There is still the CLI flag

	flagSet.StringVar(&c.KongCustomEntitiesSecret, "kong-custom-entities-secret", "", `A Secret containing custom entities for DB-less mode, in "namespace/name" format`)

I'd argue that we can add a deprecation notice and state that it doesn't do anything now and that it will be removed in next major release. WDYT?

This PR marked the flag as deprecated with flagSet.MarkDeprecated.

[rainest]

I guess we never filled out that bit of functionality in v2. we did process them in the v1 PerformUpdate call:

https://github.com/Kong/kubernetes-ingress-controller/blob/v1.3.4/internal/ingress/controller/kong.go#L37-L58

This was supposed to work in v2 also, but we forgot to add it. There seems to be little use of it in practice given that there are no user reports of it breaking. We may consider removing it as such, which would make custom entities only usable in DB-backed mode (which does not require this, since you can instead add the custom entities directly, without KIC, although they won't be KIC-managed).

I don't know if we have stats on how often we see custom plugins with custom DAOs in the wild. Anecdotally they're fairly uncommon.

[codecov]

Codecov Report

Base: 74.0% // Head: 74.1% // Increases project coverage by +0.1% 🎉

Coverage data is based on head (22dd7bd) compared to base (ef97785).
Patch coverage: 100.0% of modified lines in pull request are covered.

Additional details and impacted files

@@           Coverage Diff           @@
##            main   #3262     +/-   ##
=======================================
+ Coverage   74.0%   74.1%   +0.1%     
=======================================
  Files        110     110             
  Lines      13268   13227     -41     
=======================================
- Hits        9826    9811     -15     
+ Misses      2811    2792     -19     
+ Partials     631     624      -7     

Impacted Files	Coverage Δ
internal/dataplane/kong_client.go	85.0% <ø> (-0.1%)	⬇️
internal/dataplane/deckgen/deckgen.go	70.1% <100.0%> (+1.2%)	⬆️
internal/dataplane/sendconfig/sendconfig.go	78.1% <100.0%> (+1.1%)	⬆️
internal/manager/config.go	91.5% <100.0%> (+0.1%)	⬆️
internal/dataplane/kongstate/service.go	66.0% <0.0%> (-1.3%)	⬇️
internal/dataplane/parser/parser.go	90.7% <0.0%> (+1.6%)	⬆️
...ternal/controllers/gateway/httproute_controller.go	55.8% <0.0%> (+1.6%)	⬆️

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