don't automatically remove path_gdx_bau when 'not needed'

[orichters]

Purpose of this PR

• partly undo Check if input_bau.gdx is given or really needed; a couple of minor changes mostly related to MAgPIE coupling #1366
• Do not automatically remove path_gdx_bau, as this can be very confusing if you forget to adjust needBau when adding some call to input_bau.gdx
• Drop the check completely in readCheckScenarioConfig, as it serves no purpose anymore if you don't remove the dependency here.
• There is still a check in checkFixCfg which warns the user about it, but the removal has to be done manually.
• Drop path_gdx_bau from some config files where not needed, and fix some typos there

Type of change

• Bug fix

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

[dklein-pik]

Looks good. Thank you.