[c++/r/python] Embrace new-shape feature flag

[johnkerl]

Follow-on work to #2407.

• Remove the feature-flag detection in Python/R
• Remove feature-flag=off unit-test bits
  • Note that we still do have canned old-style experiments as .tgz so we can test old-style arrays -- nothing is lost
• Remove libtilebsoma support for schema tables with 3 rows -- insist on 5 now

This can't be done until we depend on core 2.27 since only then will we have current-domain support for dense arrays.