i18n: remove unused messages and fix comment

(1) Remove unused messages
- math-wmc-CreateRun
- math-wmc-CreateRunHelp
- math-wmc-RightSubmit
Spotted by Beta16
https://translatewiki.net/wiki/Thread:Support/About_MediaWiki:Math-wmc-CreateRun/en_and_MediaWiki:Math-wmc-CreateRunHelp/en

(2) Fix comment
- math-wmc-SectionRun
Follow-up To: I1f50792a322e52d2c9ae10cc64ff5fa5b3d55f50

(3) remove "</br>" tag

(4) fix wrong description of column header

Change-Id: I8318bd08718becd201e02a9ef9a7ddd7897fe4a7
Merge "i18n: remove unused messages and fix comment"