remove AR6 from checkProjectSummations, not needed anymore

[orichters]

Purpose of this PR

• remove AR6 from checkProjectSummations, not needed anymore
• the existence of piam_variables will still be checked by line 33

Type of change

• Remove feature

Checklist:

• My code follows the coding etiquette
• I performed a self-review of my own code
• I explained my changes within the PR, particularly in hard-to-understand areas
• I checked that the in-code documentation is up-to-date
• I adjusted the reporting in remind2 where it was needed
• I adjusted forbiddenColumnNames in readCheckScenarioConfig.R in case the PR leads to deprecated switches
• All automated model tests pass (FAIL 0 in the output of make test)
• The changelog CHANGELOG.md has been updated correctly