Remove WebView-fallback code for handling the non-functional stub.

We have already removed the non-functional WebView stub, so we can now
remove the logic ensuring that a primary device user could use any
valid WebView package even when a secondary user did not have any
WebView package to choose from (since all device users should always
have a valid package to use as WebView implementation).

Bug: 34730873
Test: run WebViewUpdateServiceTest tests.
Test: Create new user, disable Chrome for that user, ensure WebView
loads for that user and for the primary user.
Change-Id: Ia77cc2efd9e40a157fb5b68a988425ef15508033