[Fleet] Remove feature flags that are enabled by default

[kpollich]

Currently, Fleet maintains references to many feature flags that have long since been enabled by default as their corresponding features have been made generally available, e.g:

kibana/x-pack/plugins/fleet/common/experimental_features.ts

Lines 10 to 31 in 789675a

	const _allowedExperimentalValues = {
	createPackagePolicyMultiPageLayout: true,
	packageVerification: true,
	showDevtoolsRequest: true,
	diagnosticFileUploadEnabled: true,
	displayAgentMetrics: true,
	showIntegrationsSubcategories: true,
	agentFqdnMode: true,
	showExperimentalShipperOptions: false,
	agentTamperProtectionEnabled: true,
	secretsStorage: true,
	kafkaOutput: true,
	outputSecretsStorage: true,
	remoteESOutput: true,
	agentless: false,
	enableStrictKQLValidation: true,
	subfeaturePrivileges: false,
	enablePackagesStateMachine: true,
	advancedPolicySettings: true,
	useSpaceAwareness: false,
	enableReusableIntegrationPolicies: true,
	};

These should be removed as a breaking change, and their relevant code paths/conditionals should be removed to avoid needless nesting and maintenance burden on their impacted areas of the Fleet codebase.

[elasticmachine]

Pinging @elastic/fleet (Team:Fleet)

[nchaulet]

These should be removed as a breaking change, and their relevant code paths/conditionals should be removed to avoid needless nesting and maintenance burden on their impacted areas of the Fleet codebase.

Actually there is no way to disable a feature flag with a default value of true in Fleet, so I do not think it will be a breaking change to delete those.

[kpollich]

I'm removing this from our breaking changes epic as this is more just tech debt cleanup we can do at any time.