remove rarely used "Test plan:" from PR template

Created by: sqs

I've noticed that the Test plan: item in the PR template is rarely used. I would like to remove unused things to (1) reduce friction in common tasks and (2) make sure our actual practices are documented, not our ideal practices.

Only 5 of the last 33 merged PRs (excluding Renovate PRs) had test plans. Of those, only 1 actually had a true test plan, but it didn't use this template.

• https://github.com/sourcegraph/sourcegraph/pull/7103 (Test plan:Manually tested)
• https://github.com/sourcegraph/sourcegraph/pull/7098
• https://github.com/sourcegraph/sourcegraph/pull/7091 (Test plan:, i.e., it is empty, therefore not counted)
• https://github.com/sourcegraph/sourcegraph/pull/7053 (Test plan: Manually tested)
• https://github.com/sourcegraph/sourcegraph/pull/7044 (Please test the following:)
• https://github.com/sourcegraph/sourcegraph/pull/7039 (Test plan: e2e tests)