[MAINT, packaging] Remove support for briefcase installers (napari#5804)

# Fixes/Closes
Closes napari#5506

# Description

The Briefcase bundles have not been working for a while (at least in
some platforms), nobody is testing them either, and our development
efforts are mostly directed to the `constructor` installers.
In this PR, they are removed to [avoid CI
waste](napari#5660) and permit deleting
some strange code paths for Briefcase workarounds.

# References

## Type of change
- [X] Removes / deprecates code

# How has this been tested?
<!-- Please describe the tests that you ran to verify your changes. -->
- [x] No CI jobs for Briefcase
- [x] Tests pass without the extra Briefcase workarounds

## Final checklist:
- [X] My PR is the minimum possible work for the desired functionality
- [x] I have made corresponding changes to the documentation:
napari/docs#147

---------

Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>
Co-authored-by: Grzegorz Bokota <[email protected]>

.gitignore

@@ -142,11 +142,6 @@ res.qrc
 # ignore all generated themed svgs
 napari/resources/themes
 
-# briefcase
-macOS/
-linux/
-windows/
-
 napari/_version.py
 
 docs/api/napari*

MANIFEST.in

@@ -16,7 +16,6 @@ exclude napari/benchmarks/*
 recursive-exclude resources *
 recursive-exclude binder *
 recursive-exclude examples *
-exclude bundle.py
 exclude dockerfile
 exclude EULA.md
 exclude Singularity

README.md

@@ -33,7 +33,7 @@ python -m pip install "napari[all]"
 
 If you prefer conda over pip, you can replace the last line with: `conda install -c conda-forge napari`
 
-See here for the full [installation guide](https://napari.org/tutorials/fundamentals/installation.html), including how to [install napari as a bundled app](https://napari.org/tutorials/fundamentals/installation.html#install-as-a-bundled-app).
+See here for the full [installation guide](https://napari.org/tutorials/fundamentals/installation.html).
 
 ## simple example