Remove bogus TODO.

Absolute requirements of MemoryBlock include:

1) Ability to set and deny RWX permissions at the individual page level within the block
2) Ability to map and unmap from memory at will
3) Ability to specifiy the exact address at which the mapping occurs

From the docs, I don't see any way to get at any of these from the MemoryMappedFile stuff.  So that's a nonstarter.

Should be easy enough to rewrite in POSIX though.

BizHawk.Common/BizInvoke/MemoryBlock.cs

@@ -9,8 +9,6 @@ namespace BizHawk.Common.BizInvoke
 {
 	public sealed class MemoryBlock : IDisposable
 	{
-		//TODO rewrite this class without using the external functions in Kernel32 - this may help: https://docs.microsoft.com/en-us/dotnet/standard/io/memory-mapped-files
-
 		/// <summary>
 		/// starting address of the memory block
 		/// </summary>