Remove the entity_type field form Excel exports

[PiispaH]

No need to separate objects and relationships anymore.

Re #spine-tools/Spine-Toolbox#2601

Checklist before merging

• [x] Documentation (also in Toolbox repo) is up-to-date
• [x] Release notes in Toolbox repo have been updated
• [x] Unit tests have been added/updated accordingly
• [x] Code has been formatted by black
• [x] Unit tests pass

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 50.00000% with 1 lines in your changes are missing coverage. Please review.

Project coverage is 78.35%. Comparing base (ade116a) to head (d54ebe9).

Files	Patch %	Lines
spinedb_api/spine_io/importers/excel_reader.py	50.00%	0 Missing and 1 partial :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## 0.8-dev #387 +/- ## =========================================== + Coverage 78.32% 78.35% +0.03% =========================================== Files 75 75 Lines 9504 9499 -5 Branches 2015 2014 -1 =========================================== - Hits 7444 7443 -1 + Misses 1727 1723 -4 Partials 333 333 ```

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