Remove support for ./.foo.map index for ./foo mailbox.

An MH-only index of a mailbox, mbox or MMDF format, called a map file,
could be built and maintained by packf(1), rcvpack(1), and slocal(1),
but since msh(1) was removed in e6917522, there hasn't been a user of
the index file so stop producing it and delete all the supporting code
in uip/dropsbr.c.

mbx_copy() loses its `mapping' and `noisy' parameters;  noisy was only
used when mapping.  Delete mbx_read() and mbx_size() as they have no
callers without maps.