Chore: PowerToys extension development dev guide, and clean an unused msix declaration (#45967)

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## Summary of the Pull Request
PowerToys extension development dev guide, and clean an unused msix
declaration

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## PR Checklist

- [ ] Closes: #xxx
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- [ ] **Communication:** I've discussed this with core contributors
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- [ ] **Tests:** Added/updated and all pass
- [ ] **Localization:** All end-user-facing strings can be localized
- [ ] **Dev docs:** Added/updated
- [ ] **New binaries:** Added on the required places
- [ ] [JSON for
signing](https://github.com/microsoft/PowerToys/blob/main/.pipelines/ESRPSigning_core.json)
for new binaries
- [ ] [WXS for
installer](https://github.com/microsoft/PowerToys/blob/main/installer/PowerToysSetup/Product.wxs)
for new binaries and localization folder
- [ ] [YML for CI
pipeline](https://github.com/microsoft/PowerToys/blob/main/.pipelines/ci/templates/build-powertoys-steps.yml)
for new test projects
- [ ] [YML for signed
pipeline](https://github.com/microsoft/PowerToys/blob/main/.pipelines/release.yml)
- [ ] **Documentation updated:** If checked, please file a pull request
on [our docs
repo](https://github.com/MicrosoftDocs/windows-uwp/tree/docs/hub/powertoys)
and link it here: #xxx

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## Detailed Description of the Pull Request / Additional comments

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src/modules/cmdpal/ext/Microsoft.CmdPal.Ext.PowerToys/app.manifest

@@ -1,11 +1,5 @@
 <?xml version="1.0" encoding="utf-8"?>
 <assembly manifestVersion="1.0" xmlns="urn:schemas-microsoft-com:asm.v1">
-  <assemblyIdentity version="1.0.0.0" name="PowerToysExtension.app" />
-  <msix xmlns="urn:schemas-microsoft-com:msix.v1"
-        packageName="Microsoft.PowerToys.SparseApp"
-        applicationId="PowerToys.CmdPalExtension"
-        publisher="CN=PowerToys Dev, O=PowerToys, L=Redmond, S=Washington, C=US" />
-
   <asmv3:application xmlns:asmv3="urn:schemas-microsoft-com:asm.v3">
     <asmv3:windowsSettings>
       <ws2:dpiAwareness xmlns:ws2="http://schemas.microsoft.com/SMI/2016/WindowsSettings">PerMonitorV2</ws2:dpiAwareness>