chore(oidc): remove feature flag for introspection triggers (#10132)

# Which Problems Are Solved

Remove the feature flag that allowed triggers in introspection. This
option was a fallback in case introspection would not function properly
without triggers. The API documentation asked for anyone using this flag
to raise an issue. No such issue was received, hence we concluded it is
safe to remove it.

# How the Problems Are Solved

- Remove flags from the system and instance level feature APIs.
- Remove trigger functions that are no longer used
- Adjust tests that used the flag.

# Additional Changes

- none

# Additional Context

- Closes #10026 
- Flag was introduced in #7356

---------

Co-authored-by: Silvan <27845747+adlerhurst@users.noreply.github.com>

internal/feature/feature_test.go

@@ -11,7 +11,6 @@ func TestKey(t *testing.T) {
 	tests := []string{
 		"unspecified",
 		"login_default_org",
-		"trigger_introspection_projections",
 	}
 	for _, want := range tests {
 		t.Run(want, func(t *testing.T) {