Disable ScheduleCalculateWikiSizes migration and remove technical debt

What does this MR do?

In this MR we're disabling the ScheduleCalculateWikiSizes migration because it's forcing us to have some technical debt code in the ProjectStatistics code. The same migration will be removed in %14.0 by #331248 (closed).

By the time of this commit, inside %14.0 release, the migration should have already run, so it's safe to disable it.

Does this MR meet the acceptance criteria?

Conformity

• I have included a changelog entry, or it's not needed. (Does this MR need a changelog?)
• I have properly separated EE content from FOSS, or this MR is FOSS only. (Where should EE code go?)
• I have added information for database reviewers in the MR description, or it's not needed. (Does this MR have database related changes?)
• I have self-reviewed this MR per code review guidelines.
• This MR does not harm performance, or I have asked a reviewer to help assess the performance impact. (Merge request performance guidelines)
• I have followed the style guides.

Related to #240962 (closed)