Delete duplicate Buffer.Memmove implementations and avoid unnecessary pinning (#44118)

Delete duplicate Buffer.Memmove implementations and avoid unnecessary pinning (#44118)

* Delete duplicate Buffer.Memmove implementations and avoid unnecessary pinning

We have several versions of Buffer.Memmove, including one implemented around `byte*` and one implemented around `ref byte`.  We can delete the former and just use the latter everywhere.  In doing so, we can also remove a few wrapper helpers, and reduce pinning in places where those wrappers were pinning to get pointers only to eventually end up in the ref-based implementation, anyway.

* Use ThrowHelper in more places