cue/errors: remove redundant Sort call in list.sanitize

While looking at error handling code, I noticed that error list is being
sorted twice when searching for duplicates: once in sanitize,
and again in RemoveMultiples immediately after. One Sort call is enough.

Closes #2233 as merged as of commit 92b901a.

Signed-off-by: Anton Rychkov <zeithaste@gmail.com>
Change-Id: I18f8a02651e792131022e38fd870792d6bdaa33f
Reviewed-on: https://review.gerrithub.io/c/cue-lang/cue/+/552133
Unity-Result: CUEcueckoo <cueckoo@cuelang.org>
TryBot-Result: CUEcueckoo <cueckoo@cuelang.org>
Reviewed-by: Roger Peppe <rogpeppe@gmail.com>
Reviewed-by: Daniel Martí <mvdan@mvdan.cc>

cue/errors/errors.go

@@ -452,7 +452,6 @@ func (p list) sanitize() list {
 	}
 	a := make(list, len(p))
 	copy(a, p)
-	a.Sort()
 	a.RemoveMultiples()
 	return a
 }