kvdb/postgres: remove old +build directive and cleanup

Due to the update of the kvdb package to go1.18, the `// +build`
directives are no longer required. They are removed in this commit in
order to simplify upcoming commits. Along the way, a few typos are fixed
and an unused struct is removed.

kvdb/postgres/db_test.go

@@ -1,5 +1,4 @@
 //go:build kvdb_postgres
-// +build kvdb_postgres
 
 package postgres