Remove: machine_accounts feature flag.

[Mujib-Ahasan]

Summary

The default value of machine_accounts flag is false now. But related functionalities sets it to true. This PR is raised for the release of the flag.

Fixes #5435

[evankanderson]

The test failure appears to be valid:

❌ TestAssignRole/grant_permission_to_GitHub_Action (0s)
      handlers_authz_test.go:894: 
          	Error Trace:	/home/runner/work/minder/minder/internal/controlplane/handlers_authz_test.go:894
          	Error:      	Received unexpected error:
          	            	Code: 12
          	            	Name: UNIMPLEMENTED
          	            	Description: Unimplemented
          	            	Details: machine accounts are not enabled
          	Test:       	TestAssignRole/grant_permission_to_GitHub_Action
      controller.go:251: missing call(s) to *mock_roles.MockRoleService.CreateRoleAssignment(is anything, is anything, is anything, is anything, is equal to {repo:mindersec/community:ref:refs/heads/main  githubactions  } (auth.Identity), is equal to admin (authz.Role)) /home/runner/work/minder/minder/internal/controlplane/handlers_authz_test.go:839
      controller.go:251: aborting test due to missing call(s)

[evankanderson]

Ooh, this is tricky, since this is a negative use of the flag -- in this case, I think flags.Bool(ctx, s.featureFlags, flags.MachineAccounts) should effectively always return true, so if isMachine && !true will always evaluate to false, and the entire if false {...} block can be removed.