Revert "net/url, net/http/httputil: accept invalid percent encodings"

Revert "net/url, net/http/httputil: accept invalid percent encodings"

This reverts CL 450375.

Reason for revert: This change causes test failures (and possibly other
problems) for users depending on the existing validation behavior.
Rolling back the change for now to give us more time to consider its
impact. This landed late in the cycle and isn't urgent; it can wait
for 1.21 if we do want to make the change.

Fixes #56884
For #56732

Change-Id: I082023c67f1bbb933a617453ab92b67abba876ef
Reviewed-on: https://go-review.googlesource.com/c/go/+/452795
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Run-TryBot: Damien Neil <dneil@google.com>
Reviewed-by: Heschi Kreinick <heschi@google.com>