block: Remove unused BlockDeviceMapEntry

block: Remove unused BlockDeviceMapEntry

BlockDeviceMapEntry has never been used.  It was added in commit
facd6e2 "so that it is published through the introspection mechanism."
What exactly introspecting types that aren't used for anything could
accomplish isn't clear.  What "introspection mechanism" to use is also
nebulous.  To the best of my knowledge, there has never been one that
covered this type.  Certainly not query-qmp-schema, which includes
only types that are actually used in QMP.

Not being able to introspect BlockDeviceMapEntry hasn't bothered
anyone enough to complain in almost four years.  Get rid of it.

Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Eric Blake <eblake@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <20201104165513.72720-3-mreitz@redhat.com>