pg_createsubscriber: Add log message when no publications exist to drop.

pg_createsubscriber: Add log message when no publications exist to drop.

When specifying --clean=publication to pg_createsubscriber, it drops
all existing publications with a log message "dropping all existing
publications in database "testdb"". Add a new log message "no
publications found" when there are no publications to drop, making the
progress more transparent to users.

Author: Peter Smith <[email protected]>
Reviewed-by: Masahiko Sawada <[email protected]>
Discussion: https://postgr.es/m/CAHut+Ptm+WJwbbYXhC0s6FP_98KzZCR=5CPu8F8N5uV8P7BpqA@mail.gmail.com