Revert "hex: fix panic in Decode when len(src) > 2*len(dst)"

Revert "hex: fix panic in Decode when len(src) > 2*len(dst)"

This reverts CL 461958 and CL 465855.

Reason for revert: This introduced an irreconcilable inconsistency with Encode

Fixes #58391.

Change-Id: Ifd01a04d433b24c092b73e627b8149a5851c2bca
Reviewed-on: https://go-review.googlesource.com/c/go/+/469615
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Bryan Mills <bcmills@google.com>
Run-TryBot: Joseph Tsai <joetsai@digital-static.net>
Auto-Submit: Joseph Tsai <joetsai@digital-static.net>
Reviewed-by: Ian Lance Taylor <iant@google.com>