repmgr: initialise "voting_term" in "repmgr primary register"

This previously happened in the extension SQL code, which could
potentially cause replay problems if installing on a BDR cluster.

As this table is only required for streaming replication failover,
move the initialisation to "repmgr primary register".

Addresses GitHub #344 .

repmgr--4.0.sql

@@ -91,9 +91,6 @@ CREATE RULE voting_term_delete AS
    ON DELETE TO repmgr.voting_term
    DO INSTEAD NOTHING;
 
-/* XXX do this in "repmgr primary register" */
-INSERT INTO repmgr.voting_term (term) VALUES (1);
-
 
 /* ================= */
 /* repmgrd functions */