Remove API deprecated since many releases

[forman]

Removed API deprecated since many releases:

• Removed keyword argument base from function xcube.core.resampling.temporal.resample_in_time().
• Removed option base from CLI command xcube resample.
• Removed keyword argument assert_cube from xcube.core.timeseries.get_time_series().
• Removed property xcube.core.xarray.DatasetAccessor.levels.
• Removed function xcube.core.tile.parse_non_spatial_labels().
• Removed keyword argument tag from context manager xcube.util.perf.measure_time().
• Removed function xcube.core.geom.convert_geometry().
• Removed function xcube.core.geom.is_dataset_y_axis_inverted().
• Removed function xcube.util.assertions.assert_condition().
• Removed function xcube.util.cmaps.get_cmaps().
• Removed function xcube.util.cmaps.get_cmap().
• Removed function xcube.util.cmaps.ensure_cmaps_loaded().
• Removed endpoint /datasets/{datasetId}/vars/{varName}/tiles2/{z}/{y}/{x} from xcube server.

Checklist:

• [ ] ~Add unit tests and/or doctests in docstrings~
• [ ] ~Add docstrings and API docs for any new/modified user-facing classes and functions~
• [ ] ~New/modified features documented in docs/source/*~
• [x] Changes documented in CHANGES.md
• [ ] GitHub CI passes
• [ ] AppVeyor CI passes
• [x] Test coverage remains or increases (target 100%)