skip delete events of changelog table in onChangelogEvent function

[whhe]

A Pull Request should be associated with an Issue.

Related issue: https://github.com/github/gh-ost/issues/1427

Description

This PR skip the DELETE events of changelog table in onChangelogEvent function.

In case this PR introduced Go code changes:

• [x] contributed code is using same conventions as original code
• [x] script/cibuild returns with no formatting errors, build errors or unit test errors.

[whhe]

Replaced by #1434, closed.