Don't use SearchEngineConfig::searchableNamespaces in User::getDefaultOptions.

Don't use SearchEngineConfig::searchableNamespaces in User::getDefaultOptions.

Default options should be the same for all users. SearchEngineConfig::searchableNamespaces
however calls a hok that allows the set of searchable namespaces to be adjusted per user,
e.g. based on the user groups or permissions, like Extension:Lockdown does.

Since SearchableNamespace hook handlers may access the global user objects, problems arise
when it is that global user object trying to initialize itself that triggers the call to
User::getDefaultOptions. This can cause recursive calls to User::load(), see I6d1b9fe07.

Furthermore, these seems to be no need to actively record the searchable namespaces beyond
the contents of $wgNamespacesToBeSearchedDefault. If a 'searchNs' option is absent, it is
treated as disabled.

Bug: T142295
Bug: T137051
Change-Id: I5f6bcdfc588acef0873136bf338d79890863e009