`delete-windows-on' completion list should include only buffers that correspond to FRAME arg

[Drew Adams]

The completion list of command `delete-windows-on' is all buffers -
`interactive' is simply called with "bDelete windows on (buffer): ". Most of
the time, however, many of those buffers have no windows to delete.

It would be better to have `interactive' check the FRAME arg, and propose a
list of buffer candidates for completion that is compatible (consistent)
with FRAME.