cmd/compile: remove unnecessary error condition on reading fingerprint

io.ReadFull guarantees n == len(buf) if and only if err == nil,
so the length check is redundant.

Change-Id: I15bff97868e27a65648acd791883cac8dab77630
Reviewed-on: https://go-review.googlesource.com/c/go/+/232988


Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>