Simplify db_fetch_array()

Since all the ADOdb drivers we use now have proper support for
associative fetch mode including ability to force conversion of field
names to lowercase, we get rid of the special handling we had for oci8
and pgsql.

We still need some specific processing:

• pgsql: convert boolean ('t'/'f') to PHP bool (true/false)
• oci8: convert null values to empty strings