refactor db_subscr_purge

Use named parameters in the SQL statements.

Use db_bind_* functions to drop some code dup.

Adopt error handling (rc and logging) to match the other db functions: return
-ENOENT for unknown subscriber, -EIO for SQL failures.

Change-Id: Iad49d29b90a708c6cf55bfb3bcc02d9e29001a15
6 files changed