Remove package lock files.

We weren't using RestoreLockedMode in CI, so these were pointless. And that feature doesn't work cross-platform (NuGet/Home#9195) so enabling it breaks the builds anyway.

Author: bgrainger

Directory.Build.props

@@ -9,7 +9,6 @@
     <NuGetAudit>true</NuGetAudit>
     <NuGetAuditLevel>low</NuGetAuditLevel>
     <NuGetAuditMode>all</NuGetAuditMode>
-    <RestorePackagesWithLockFile>true</RestorePackagesWithLockFile>
   </PropertyGroup>
 
   <PropertyGroup Condition=" '$(Configuration)' == 'Release' ">