remove deprecated Base.Result.ok_unit call
remove deprecated Base.Result.ok_unit call

Summary: `ok_unit` was a "performance hack" equivalent to `Ok ()`, but was deprecated in Base v0.13.0 because `Ok ()` is also statically allocated

Reviewed By: dsainati1

Differential Revision: D21565989

fbshipit-source-id: fa33447d0511a26c89d28338a5e5e33f7e851c79

nmote/flow,nmote/flow,nmote/flow,nmote/flow,nmote/flow,nmote/flow,nmote/flow