[path_provider] Change Linux example to depend on endorsed plugin from pub.dev

[TimWhiting]

Description

This is a simple pull request to update the dependencies of the path_provider_linux example to depend on the endorsed plugin, rather than a local path.

I also took the liberty of removing the example/ios and example/android folders, since those are no longer required. (#2804)

Related Issues

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[TimWhiting]

@stuartmorgan
Ready for review. This is just updating the linux example to depend on endorsed plugin, and removing the now unnecessary ios and android folders from the example.

[stuartmorgan]

Thanks for the cleanup! Just one issue.

[TimWhiting]

@stuartmorgan
Done!

[stuartmorgan]

LGTM!