Convert calls to db_placeholders() in static queries

Moving this over from #314464: Convert db_placeholders() to DBTNG

This patch takes all occurrences of db_placeholders() in static queries and makes them use just normal dbtng placeholders which now support arrays.

It doesn't touch dynamic queries built with concatenated strings, nor queries which are wrapped in db_rewrite_sql() - these will need to be converted to db_select() anyway.