Use CoreCLR's default managed structure layout algorithm

[joshpeterson]

Unity currently requires that managed structures have a fixed, known layout. We're making changes to CoreCLR to support this layout requirement, but longer term, we want to use the default CoreCLR layout algorithm, as that is tuned for better GC performance.

[joshpeterson]

Since we plan to change the way Unity works with managed structures in native code in order to use the CoreCLR GC without modification (see #16), we won't need to change the layout algorithm. So we don't need to do this.