CLN: address FIXME/TODO/XXX comments
- changed XXX comments to TODO (much more grep-friendly, since we sometimes use XXX for a `name` attribute.
- Removed some now-fixed FIXME/TODO comments
  - tests.arithmetic.test_datetime64, tests.arithmetic.test_period, test_conversion, tests.indexes.common, test_date_range, test_css, scalar.timedelta.test_arithmetic
- Added some TODO(EA2D) comments
- Removed some `TODO(wesm): unused?` comments