[Test][stdlib] Remove obsolete i386 UNSUPPORTED directive

The test was disabled on i386 because of
#52252.
That bug is fixed, so the comments can be removed.

Author: ramonasuncion

test/stdlib/KVOKeyPaths.swift

@@ -6,10 +6,6 @@
 
 // REQUIRES: objc_interop
 
-// FIXME: https://github.com/apple/swift/issues/52252
-// Disable because it blocks PR testing.
-// UNSUPPORTED: CPU=i386
-
 import Foundation
 
 struct Guts {