Don't change asyncpg's "char" codec

Don't change asyncpg's "char" codec

This codec was used to ensure the "pg_attribute.generated"
column comes back as a string and not bytes, matching how
other PG drivers treat this datatype.   However, this breaks
asyncpg's internal implementation of set_type_codec going forward
and the "char" datatype is actually a bytes in any case.
So at the moment it appears psycopg2/pg8000 are broken for mis-treatment
of the datatype and asyncpg is broken in that it was allowing
us to change a codec that it appears to rely upon internally.

Fixes: #5586
Change-Id: I937eba315904721aa4e2726b95432910a8affe5f