[release-branch.go1.14] database/sql: de-flake TestTxCannotCommitAfterRollback

Do not cancel rows during test. Only cancel the Tx.
Correct the referenced issue number on the test.

Updates #38597.
Fixes #41815.

Change-Id: I0e8ba1bf2a8ba638d121c9c6938501fec1d5e961
Reviewed-on: https://go-review.googlesource.com/c/go/+/229478


Run-TryBot: Daniel Theophanes <kardianos@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
(cherry picked from commit ed7888ae)
Reviewed-on: https://go-review.googlesource.com/c/go/+/259858


Trust: Dmitri Shuralyov <dmitshur@golang.org>
Trust: Emmanuel Odeke <emm.odeke@gmail.com>
Run-TryBot: Dmitri Shuralyov <dmitshur@golang.org>
Reviewed-by: Emmanuel Odeke <emm.odeke@gmail.com>
TryBot-Result: Go Bot <gobot@golang.org>