Replace os() with canImport(Darwin) (#693)

* Replace os() with canImport(Darwin)

* Remove compilation conditional since it'll always be true

Author: o-nnerb

Sources/AsyncHTTPClient/Base64.swift

@@ -146,11 +146,6 @@ extension String {
   }
 }
 
-// Frustratingly, Swift 5.3 shipped before the macOS 11 SDK did, so we cannot gate the availability of
-// this declaration on having the 5.3 compiler. This has caused a number of build issues. While updating
-// to newer Xcodes does work, we can save ourselves some hassle and just wait until 5.4 to get this
-// enhancement on Apple platforms.
-#if (compiler(>=5.3) && !(os(macOS) || os(iOS) || os(tvOS) || os(watchOS))) || compiler(>=5.4)
 extension String {
 
   @inlinable
@@ -163,12 +158,3 @@ extension String {
     }
   }
 }
-#else
-extension String {
-  @inlinable
-  init(customUnsafeUninitializedCapacity capacity: Int,
-     initializingUTF8With initializer: (_ buffer: UnsafeMutableBufferPointer<UInt8>) throws -> Int) rethrows {
-    try self.init(backportUnsafeUninitializedCapacity: capacity, initializingUTF8With: initializer)
-  }
-}
-#endif