RSpec: Configure `focus` in a single location `support/rspec`

What does this MR do and why?

Because support/rspec is loaded by all spec helpers we can define the focus configuration only once and delete other definitions. This makes the spec helpers a tad slimmer.

Contributes to #372849.

How to set up and validate locally

# Add :focus tag to a `describe`, `context` or `it`

bin/rspec <fast_spec>
bin/rspec <spec>


# It only runs the focused spec.

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