fmt: don't pad strings with zeros

fmt: don't pad strings with zeros

It's what the documentation says, and oddly it already behaves correctly
for right padding, not left. (We never pad with zeros on the right.)

Just don't do it.

Fixes #56486

Change-Id: I2465edea93c69084e33bee0d945d5a1b85e6cd14
Reviewed-on: https://go-review.googlesource.com/c/go/+/555776
Reviewed-by: Russ Cox <rsc@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Rob Pike <r@golang.org>
Reviewed-by: Cherry Mui <cherryyz@google.com>