drop support for user-defined databases

[kevinushey]

Pull Request Template for Rcpp

Closes #1313.

Checklist

• Code compiles correctly
• R CMD check still passes all tests
• Preferably, new tests were added which fail without the change
• Document the changes by file in ChangeLog

[eddelbuettel]

Nice catch in #1313. I don't even recall what those user-define dbs were....

[eddelbuettel]

I am kinda minutes/hours/days from a new release so how would you feel if I delayed this til after the release. Given that @mattfidler saw this I would think we need a full rev.dep check (as we change the API surface) which I would rather avoid now. Thoughts?

[kevinushey]

For sure, definitely no rush on this (or the other PR I just put up).

[mattfidler]

For me I worked around it, if CRAN does ask for an update I am ready.

I dont mind waiting either.

[eddelbuettel]

Reverse dependency check is now 50% done, no new regressions yet.

[eddelbuettel]

The reverse depends run had no new issues, so merging this now. Had to do a micro-commit over conflicts in ChangeLog, will do one more clean-up (fixing tabs/spaces etc) once merged and once I increment the micro release.