Revert "encoding/xml: fix reserved namespace check to be case-insensitive"

This reverts CL 203417.

Reason for revert: This change changes uses of tags like "XMLSchema-instance" without any recourse.

For #35151
Fixes #39876

Change-Id: I4c85c8267a46b3748664b5078794dafffb42aa26
Reviewed-on: https://go-review.googlesource.com/c/go/+/240179


Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Emmanuel Odeke <emm.odeke@gmail.com>
Reviewed-by: Andrew Bonventre <andybons@golang.org>