Remove -ruler.query-frontend.timeout and use -querier.timeout instead

[pracucci]

#2090 introduced -ruler.query-frontend.timeout. We're trying to reduce config options, so an alternative option could be reusing -querier.timeout, documenting that needs to be set for ruler too when remote rule evaluation is used. We already use -querier.timeout in query-frontend too.

[pstibrany]

I think we should fix this before 2.2 release, so that we don't release new flag (-ruler.query-frontend.timeout) to the world.

[stevesg]

Doing it now :)