Revert "reflect: support multiple keys in struct tags"

Proposal #40281 was initially accepted, but has now been declined.
This CL removes most of the work done to implement it.

Specifically this reverts CLs 248341, 274448, 274474, and 278392.

For #40281
For #43226

Change-Id: I5a9ebb4d9cb5fb0962434b64c59beb8343030be5
Reviewed-on: https://go-review.googlesource.com/c/go/+/281515


Trust: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Emmanuel Odeke <emmanuel@orijtech.com>