Fix override of content view page provider by a generic page provider

Steps to reproduce:

• define a content view using a standard core page provider
• override it with a generic page provider

=> the core page provider is still used

Note that the content view logic to retrieve the first enabled page provider definition it holds is correct, but merging logic relies on each page provider enablement => it's not possible to remove or disable the initial core page provider to make merging keep only the new generic page provider.