remove snapshot support

[pfuehrlich-pik]

Purpose of this PR

Renv now fully supersedes snapshots, so this PR removes all snapshot related code.

Type of change

• Refactoring

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)

Further information (optional):

• Test runs are here:
• Comparison of results (what changes by this PR?):

[dklein-pik]

I'd prefer to keep the snapshot functionality until the next MAgPIE release that also supports renv.