autoconvert: Remove the initial-identity property from autoconvert

autoconvert: Remove the initial-identity property from autoconvert

Initially creating an identity element to forward serialized
events downstream before any caps are known is broken behaviour.

Serialized events should only be forwarded downstream if the
caps are already known, otherwise autopluggers and other elements
using pad-blocks will fail.

This behaviour also doesn't work anymore after basetransform
was fixed to queue serialized events until the caps are known
as a result of fixing bug #659571.

See bug #599469, #665205.