api: remove unused singular option
api: remove unused singular option

The singular option to the API language was added as a way to deal with messages
that do not have a reply message. Examples in memclnt.api.
Instead dealt with these messages using the service {} construct.

Type: refactor
Signed-off-by: Ole Troan <aeed71d78dc09e4764ceac69b111eee9b50cfdd0@cisco.com>
Change-Id: If01b390b24b7539d1f93de4b8edfe1dad08e509d

FDio/vpp,FDio/vpp,FDio/vpp,FDio/vpp,FDio/vpp,chrisy/vpp,chrisy/vpp,chrisy/vpp,chrisy/vpp,FDio/vpp,chrisy/vpp,chrisy/vpp,chrisy/vpp,chrisy/vpp,FDio/vpp,FDio/vpp