Remove obsolete tests from System.Net.Security (#624)

Remove obsolete tests from System.Net.Security (#624)

These tests haven't been running nor even compiling for several years. The compilation
broke when CSPROJ changes were made regarding TargetsLinux vs. TargetsUnix.

The tests themselves required running in admin mode and made impactful changes on the
host machine (such as installing a KDC).

Now that enterprise scenario tests have been added with PR #463, these tests are no longer
needed. Much of these tests have already been incorporated into the new enterprise
tests.

Closes https://github.com/dotnet/corefx/issues/30150
Closes https://github.com/dotnet/corefx/issues/24707