swiftPackages.swift-unwrapped: drop unnecessary `mkdir`

Honestly I did this in the middle of trying things and then forgot
about it, and I just don’t feel like bootstrapping the whole thing
again to drop this commit.

pkgs/development/compilers/swift/compiler/default.nix

···
     mv llvm/bin/clang-15{-unwrapped,}
     mv swift/bin/swift-frontend{-unwrapped,}
 
-    mkdir $out $lib
+    mkdir $lib
 
     # Install clang binaries only. We hide these with the wrapper, so they are
     # for private use by Swift only.