[main] Delete security-partner-dotnet pipeline (#18502)

Co-authored-by: Michael Simons <msimons@microsoft.com>

src/SourceBuild/tarball/content/eng/pipelines/security-partners-dotnet.yml

@@ -1,32 +0,0 @@
-trigger: none
-
-variables:
-- name: cfsNPMWarnLevel
-  value: none
-
-- name: cfsNugetWarnLevel
-  value: none
-
-- name: myGetWarnLevel
-  value: none
-
-- name: NuGetSecurityAnalysisWarningLevel
-  value: none
-
-jobs:
-- template: ../../src/installer/src/SourceBuild/Arcade/eng/common/templates/job/source-build-build-tarball.yml
-  parameters:
-    architecture: x64
-    excludeSdkContentTests: true
-    matrix:
-      Ubuntu2004-Offline:
-        _BootstrapPrep: true
-        _Container: mcr.microsoft.com/dotnet-buildtools/prereqs:ubuntu-20.04
-        _EnablePoison: false
-        _ExcludeOmniSharpTests: false
-        _RunOnline: false
-        _WithPreviousSDK: false
-    name: Build_Tarball_x64
-    pool:
-      name: NetCore1ESPool-Svc-Internal
-      demands: ImageOverride -equals Build.Ubuntu.1804.Amd64