Remove a conditional that can never trigger

What does this MR do and why?

Describe in detail what your merge request does and why.

Removes a conditional that never triggers, and that references a constant that doesn't exist.

The constant used to exist in https://gitlab.com/gitlab-org/gitlab/-/blob/d96585f5739f4cb83fd00fa402192a15d6958881/app/models/global_milestone.rb#L7 but was missed when moving the code over to a new class. However it never raises an error because the conditional can never trigger as projects is a required parameter, and is seemingly never called with nil in the codebase. An empty array will still return the default values below anyway.

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