R1:64886b11d8d1

Remove expensive, pointless typeachecking in custom fields

Summary:
Ref T11404. On my system, this improves performance by 10-15% for `differential.revision.search`.

`PhutilTypeSpec` provides high quality typechecking and is great for user-facing things that need good error messages.

However, it's also a bit slow, and pointless here (the API is internal and it only has one possible option).

I think I added this after writing `checkMap` just because I wanted to use it more often. My desire is sated…