Chore: remove session storage references (#16445)

* Chore: remove session storage references

* Small refactoring of the settings module

* Update docs - remove references for the session storage

* Update config files (sample and default configs)

* Add tests for warning during the config load on defined storage cache

* Remove all references to session storage

* Remove macaron session dependency

* Remove leftovers

* Fix: address review comments

* Fix: remove old deps

* Fix: add skipStaticRootValidation = true to tests

* Fix: improve the docs and warning message

As per discussion in here - https://github.com/grafana/grafana/pull/16445/files#r273026255

* Chore: make linter happy

Fixes #16148
Ref #16114

pkg/models/context.go

@@ -4,7 +4,6 @@ import (
 	"strings"
 
 	"github.com/grafana/grafana/pkg/log"
-	"github.com/grafana/grafana/pkg/services/session"
 	"github.com/grafana/grafana/pkg/setting"
 	"github.com/prometheus/client_golang/prometheus"
 	"gopkg.in/macaron.v1"
@@ -15,9 +14,6 @@ type ReqContext struct {
 	*SignedInUser
 	UserToken *UserToken
 
-	// This should only be used by the auth_proxy
-	Session session.SessionStore
-
 	IsSignedIn     bool
 	IsRenderCall   bool
 	AllowAnonymous bool