Click hook should not allow more than a provider per plugin

The new click hook expects to find a "plugins" key in the JSON manifest file for Online Accounts, which would contain a list of plugins.
However, the click manifest already allows a click package to ship more applications and account plugins in a single .click file, so this functionality is not needed. On the contrary, it's undesirable because it adds one more token to the provider name ("<package>_<plugin>_<provider>" instead of just "<package>_<plugin>").

Removing this extra token also has the advantage that we can keep exactly the same provider ID we had with the old hooks, therefore minimizing changes for applications.