encoding/xml: move unexported const out of exported const block

encoding/xml: move unexported const out of exported const block

CL 58210 introduced this constant for reasons I don't understand.
It should not be in the exported const block, which will pollute
godoc output with a "... unexported" notice.

Also since we already have a constant named xmlnsPrefix for "xmlns",
it is very confusing to also have xmlNamespacePrefix for "xml".
If we must have the constant at all, rename it to xmlPrefix.

Change-Id: I15f937454d730005816fcd32b1acca703acf1e51
Reviewed-on: https://go-review.googlesource.com/78121
Run-TryBot: Russ Cox <rsc@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>