Revert "database/sql: allow drivers to override Scan behavior"

Revert "database/sql: allow drivers to override Scan behavior"

This reverts CL 588435.

This new API is difficult to use correctly, and in many cases cannot
be used efficiently. We're going to work on this problem a bit more.

The release notes are removed by CL 729340, since they were moved to
the x/website repository since the original CL was made.

Reopens #67546.

Change-Id: I2a2bd25f2fce5f02e4d28cd33a9cc651bf35ab50
Reviewed-on: https://go-review.googlesource.com/c/go/+/729360
Reviewed-by: Damien Neil <dneil@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Sean Liao <sean@liao.dev>