Remove unnecessary [NotNull] attributes

When subclassing `Command<TSettings>` which has the [NotNull] attributes, Rider/ReSharper gives this warning:
> Nullability of type of parameter 'context' in method does not match overridden member `int Spectre.Console.Cli.Command<TSettings>.Execute(CommandContext, TSettings)` (possibly because of nullability attributes)

When subclassing `Command<TSettings>` which does not have the [NotNull] attributes, Rider/ReSharper gives this warning:
> The nullability attribute has no effect and can be safely removed

The solution is simply to remove the [NotNull] attributes.

Since `<Nullable>enable</Nullable>` is set in the project, they are actually not necessary. By the way, the non-generic `Command` class does not have the [NotNull] attributes.

src/Spectre.Console.Cli/Internal/Commands/ExplainCommand.cs

@@ -34,7 +34,7 @@ internal sealed class ExplainCommand : Command<ExplainCommand.Settings>
         public bool IncludeHidden { get; }
     }
 
-    public override int Execute([NotNull] CommandContext context, [NotNull] Settings settings)
+    public override int Execute(CommandContext context, Settings settings)
     {
         var tree = new Tree("CLI Configuration");
         tree.AddNode(ValueMarkup("Application Name", _commandModel.ApplicationName, "no application name"));

src/Spectre.Console.Cli/Internal/Commands/VersionCommand.cs

@@ -15,7 +15,7 @@ internal sealed class VersionCommand : Command<VersionCommand.Settings>
     {
     }
 
-    public override int Execute([NotNull] CommandContext context, [NotNull] Settings settings)
+    public override int Execute(CommandContext context, Settings settings)
     {
         _writer.MarkupLine(
             "[yellow]Spectre.Cli[/] version [aqua]{0}[/]",

src/Spectre.Console.Cli/Internal/Commands/XmlDocCommand.cs

@@ -17,7 +17,7 @@ internal sealed class XmlDocCommand : Command<XmlDocCommand.Settings>
     {
     }
 
-    public override int Execute([NotNull] CommandContext context, [NotNull] Settings settings)
+    public override int Execute(CommandContext context, Settings settings)
     {
         _writer.Write(Serialize(_model), Style.Plain);
         return 0;