cmd/compile/internal/gc: remove dead code from stringtoarraylit

cmd/compile/internal/gc: remove dead code from stringtoarraylit

The code path for []byte is unused.
Rename function to stringtoruneslit to reflect change in the behavior.

Note that removed code had a bug in it,
it used [0] index instead of [i] inside a loop body.

Change-Id: I58ece5d9d3835887b014446f8a7d3e7fc2fdcaa3
Reviewed-on: https://go-review.googlesource.com/c/125796
Run-TryBot: Iskander Sharipov <iskander.sharipov@intel.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>