Remove MemoryExtensions.Slice (for now)

[stephentoub]

#37742 (comment)
cc: @GrabYourPitchforks

[Dotnet-GitSync-Bot]

I couldn't figure out the best area label to add to this issue. Please help me learn by adding exactly one area label.

[GrabYourPitchforks]

@stephentoub I assume this tracks #37757, or was there a different API we should temporarily back out?

[stephentoub]

Oh, that PR passed by without me even noticing :)