crimson/osd: remove extraneous co_return

crimson/osd: remove extraneous co_return

no need to use co_return at the end of a coroutine which returns
future<>. despite that this does not incur performance penalty,
because:

co_await func() // func returns future<>

returns void

co_return is called with void operand, this statement is lowered to

promise.return_void()

and a bare "co_await func()" also calls return_void(). so they are
equivalent at runtime. but the extraneous co_return can be dispensed.

also take this opportunity to remove the bare co_return at the end
of a coroutine. it's unnecessary just like a `return` at the end of
a function returning `void`.

Signed-off-by: Kefu Chai <k.chai@proxmox.com>