[test] Re-enable test whose issue has long since been fixed (#21789)

Unfortunately deserialize_appkit.sil seems to have developed another
issue in the mean time. Filed rdar://problem/47200045 for that.

Author: jrose-apple

test/SIL/Serialization/deserialize_foundation.sil

@@ -5,6 +5,3 @@
 // REQUIRES: objc_interop
 
 // CHECK-NOT: Unknown
-
-// FIXME: rdar://23805004 Non-fragile shared functions are incorrectly being referenced in fragile functions
-// REQUIRES: rdar23805004