[BUGFIX] Add domain model validators only once

If a domain model validator is present and a controller action accepts
this domain model as argument, the validator is added twice to the
validation tree.

It is added in ValidatorResolver::buildMethodArgumentsValidatorConjunctions()
and in ValidatorResolver::addCustomValidators() which is called from
ValidatorResolver::getBaseValidatorConjunction()

Both methods are called when building the validation tree in
ActionController::initializeActionMethodValidators()

Resolves: #49624
Releases: master, 2.1, 2.0
Change-Id: I1f8902d09f468177d580a7a3d949242cce78313e