Remove dead-lettered Centos.7.Amd64.Open queue (#26238)

- was breaking Helix full matrix tests temporarily
- more importantly, queue is redundant given RHEL coverage

Author: dougbu

eng/targets/Helix.Common.props

@@ -32,7 +32,6 @@
     <HelixAvailableTargetQueue Include="Windows.10.Amd64.Server20H1.Open" Platform="Windows" />
     <HelixAvailableTargetQueue Include="Ubuntu.1804.Amd64.Open" Platform="Linux" />
     <HelixAvailableTargetQueue Include="Ubuntu.2004.Amd64.Open" Platform="Linux" />
-    <HelixAvailableTargetQueue Include="Centos.7.Amd64.Open" Platform="Linux" />
     <HelixAvailableTargetQueue Include="Debian.9.Amd64.Open" Platform="Linux" />
     <HelixAvailableTargetQueue Include="Redhat.7.Amd64.Open" Platform="Linux" />
     <HelixAvailableTargetQueue Include="(Fedora.28.Amd64.Open)Ubuntu.1604.Amd64.Open@mcr.microsoft.com/dotnet-buildtools/prereqs:fedora-28-helix-09ca40b-20190508143249" Platform="Linux" />