[cleanup][misc] Delete .github/ISSUE_TEMPLATE/pip.md

[asafm]

Motivation

No need for that anymore. We have moved to submitting PIPs using PRs.

Modifications

Delete issue template named PIP.

Verifying this change

• Make sure that the change passes the CI checks.

(Please pick either of the following options)

This change is a trivial rework / code cleanup without any test coverage.

Does this pull request potentially affect one of the following parts:

If the box was checked, please highlight the changes

• Dependencies (add or upgrade a dependency)
• The public API
• The schema
• The default values of configurations
• The threading model
• The binary protocol
• The REST endpoints
• The admin CLI options
• The metrics
• Anything that affects deployment

Documentation

• doc
• doc-required
• doc-not-needed
• doc-complete

[asafm]

@codelipenghui @tisonkun Take a look

[tisonkun]

LGTM. I'll merge in days if there is no objection.