Remove manifest that no longer works for Firefox Nightly version 81.0a1 (#5737)

manifests/Mozilla/FirefoxNightly/81.0a1.yaml

@@ -1,18 +0,0 @@
-Id: Mozilla.FirefoxNightly
-Version: 81.0a1
-Name: Firefox Nightly
-Publisher: Mozilla
-License: Mozilla Public License V2.0
-LicenseUrl: https://www.mozilla.org/en-US/MPL/2.0/
-AppMoniker: firefox-nightly
-Tags: firefox, browser, mozilla, nightly
-Description: "Get a sneak peek at our next generation web browser, and help us make it the best browser it can be: try Firefox Nightly."
-Homepage: https://www.mozilla.org/en-US/firefox/nightly/
-Installers:
-  - Arch: x64
-    Url: https://download-installer.cdn.mozilla.net/pub/firefox/nightly/latest-mozilla-central/firefox-81.0a1.en-US.win64.installer.msi
-    Sha256: ABC41D398BFCAC25DFE01F45B7702C1D7311E8CF244C13CD8D1F605C87BB6359
-    InstallerType: msi
-    Switches:
-      Silent: /quiet /norestart
-      SilentWithProgress: /passive /norestart