Remove System.Security.Permissions.SecurityPermissionAttribute from System.Private...

Remove System.Security.Permissions.SecurityPermissionAttribute from System.Private.CoreLib.

When compiling with /unsafe+ Roslyn emits
  .permissionset reqmin
             = {class 'System.Security.Permissions.SecurityPermissionAttribute, System.Private.CoreLib,
             Version=4.0.0.0, Culture=neutral, PublicKeyToken=7cec85d7bea7798e' =
             {property bool 'SkipVerification' = bool(true)}}
if System.Security.Permissions.SecurityPermissionAttribute is present.

In particular, System.Private.CoreLib.dll is compiled with /unsafe+ and has the above in the manifest.

https://github.com/dotnet/coreclr/commit/e516211b2d3a3ae2dbd80f2c94b129910a9e280d
made BclRewriter remove System.Security.Permissions.SecurityPermissionAttribute
from System.Private.CoreLib.dll but the above reference remained.

This change removes System.Security.Permissions.SecurityPermissionAttribute completely
to avoid confusion when tools are analyzing msil and try to resolve the manifest
of System.Private.CoreLib.dll.