Remove unneeded backports

Sources/_SwiftSupport.swift

@@ -1,42 +1,5 @@
 import Foundation
 
-#if !swift(>=4.1)
-  public extension Sequence {
-    func compactMap<ElementOfResult>(_ transform: (Element) throws -> ElementOfResult?) rethrows -> [ElementOfResult] {
-      return try flatMap(transform)
-    }
-  }
-#endif
-
-#if !swift(>=4.1)
-  public extension Collection {
-    func index(_ index: Self.Index, offsetBy offset: Int) -> Self.Index {
-        let indexDistance = Self.IndexDistance(offset)
-        return self.index(index, offsetBy: indexDistance)
-    }
-  }
-#endif
-
-#if !swift(>=4.1)
-public extension TemplateSyntaxError {
-  public static func == (lhs: TemplateSyntaxError, rhs: TemplateSyntaxError) -> Bool {
-    return lhs.reason == rhs.reason &&
-      lhs.description == rhs.description &&
-      lhs.token == rhs.token &&
-      lhs.stackTrace == rhs.stackTrace &&
-      lhs.templateName == rhs.templateName
-  }
-}
-#endif
-
-#if !swift(>=4.1)
-public extension Variable {
-  public static func == (lhs: Variable, rhs: Variable) -> Bool {
-    return lhs.variable == rhs.variable
-  }
-}
-#endif
-
 #if !swift(>=4.2)
 extension ArraySlice where Element: Equatable {
     func firstIndex(of element: Element) -> Int? {