[stdlib] Removing the deprecated flatMap overload (#9397)

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>

Author: moiseev

stdlib/public/core/SequenceAlgorithms.swift.gyb

@@ -729,13 +729,6 @@ extension Sequence {
     }
     return result
   }
-
-  @available(*, deprecated, message: "This call uses implicit promotion to optional. Please use map instead.")
-  public func flatMap<T>(
-    _ transform: (${GElement}) throws -> T
-  ) rethrows -> [T] {
-    return try map(transform)
-  }
 }
 
 extension Sequence {

test/Constraints/closures.swift

@@ -512,8 +512,7 @@ func returnsArray() -> [Int] { return [] }
 
 returnsArray().flatMap { $0 }.flatMap { }
 // expected-warning@-1 {{expression of type 'Int' is unused}}
-// expected-warning@-2 {{Please use map instead.}}
-// expected-warning@-3 {{result of call to 'flatMap' is unused}}
+// expected-warning@-2 {{result of call to 'flatMap' is unused}}
 
 // rdar://problem/30271695
 _ = ["hi"].flatMap { $0.isEmpty ? nil : $0 }