[IO][NFC] Remove unnecessary if/else
[IO][NFC] Remove unnecessary if/else

Since commit 65bbfec1ecf until commit ad568ea83b0, a non-zero `len`
could be passed as a parameter of STLKind, in which case STLKind
only looked at the first `len` characters to determine the result.

From commit ad568ea83b0, `len` is always set to `type.length()`,
and therefore it's never zero in well-formed programs.

root-mirror/root,root-mirror/root,root-mirror/root,root-mirror/root,root-mirror/root,olifre/root,root-mirror/root,olifre/root,olifre/root,root-mirror/root,olifre/root,olifre/root,olifre/root,olifre/root,olifre/root,olifre/root,olifre/root,olifre/root,root-mirror/root,root-mirror/root,root-mirror/root,root-mirror/root