[TASK] Remove array duplication in T3_SERVICES

These informations are important for a service:

* Extension name of a service implementation (e.g. 'saltedpasswords')
* Service type (eg. 'auth')
* Class implementing the service
  (service key, e.g. 'tx_saltedpasswords_sv1').

t3lib_extMgm::addService() registers a service in global array
$T3_SERVICES[type][key].
Additionally it registers the same service as $T3_SERVICES[key][key].

The latter registration is actually never used by any other service
method in the core and can be removed to simplify this handling.

Change-Id: I2c8c53bb763b0a41e5c2488bbcef96f7e1505c9f
Resolves: #29164
Reviewed-on: http://review.typo3.org/4522
Reviewed-by: Steffen Gebert
Tested-by: Steffen Gebert
Reviewed-by: Susanne Moog
Tested-by: Susanne Moog