fix($injector): remove INSTANTIATING entry when done.

getService flags services as INSTANTIATING while it calls their
provider factory, in order to detect circular dependencies. If
the service is instantiated correctly, the INSTANTIATING flag is
overwritten with the actual service. However, if the service is
not instantiated correctly, the INSTANTIATING flag should still
be removed, or all further requests for this service will be
mis-detected as a circular dependency.

Closes #4361.