Code Quality: Clean solution

[Lamparter]

I noticed that the rules proposed in #11346 had not exactly been reflected properly across the codebase. In this PR I cleaned source files by removing unnecessary using directives (sparing those in GlobalUsings.cs files, of course) and more importantly enforced tabs across the source, in compliance with the code styles this repository demands.

Resolved / Related Issues

• Related Enforce tabs for Xaml in EditorConfig and Xaml Styler #11346

This PR is a relatively simple change and so does not require a linked issue. If the reviewer would like, I will create an issue to discuss the changes.

Steps used to test these changes

This PR does not modify the app's internal codebase in a breaking way.

[Lamparter]

Ouch, that's not good...

Error: D:\a\Files\Files\src\Files.App\ViewModels\UserControls\Previews\ShellPreviewViewModel.cs(32,3): error CS0246: The type or namespace name 'ContentExternalOutputLink' could not be found (are you missing a using directive or an assembly reference?) [D:\a\Files\Files\src\Files.App\Files.App.csproj]

[0x5bfa]

What on earth are you actually doing?
I just told you to create a tracking issue that has been approved or get an approval from Yair on Files Discord right before.

Please DO NOT mix up changes here, it wastes "our time" to spend on reviewing large PRs.

• Tab fixes in these files are done in another PR #15948
• Removal of unnecessary usings should not be don't in a separate PR. Just each author of PRs should take care of it when they're working on these files.
• we add/delete global usings very carefully not to cause ambiguity.

[Lamparter]

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