[stdlib] Removing the deprecated flatMap overload

[moiseev]

The following code behaves incorrectly due to the presence of this
overload.

let a: Int = 1
let b: Int? = 2
let c: Int? = nil
let result: [Any] = [a, b, c].flatMap { $0 }

Fixes: rdar://problem/31910642

• Explanation: A deprecated overload for flatMap behaves incorrectly in presence of Any type
• Scope of Issue: The deprecated overload was added as a hint for developers that they might be using an inefficient call to flatMap instead of a more efficient map. It’s removal will fix the problem without breaking anything.
• Risk: Minimal
• Reviewed By: Ben Cohen
• Testing: Automated test suite
• Directions for QA: N/A
• Radar: rdar://problem/32054458

[moiseev]

@swift-ci Please test

[moiseev]

Similar to #7823