Remove dead code from DBI::Replicated

Remove dead code from DBI::Replicated

This was originally introduced in fecb38cba, without tests nor justification.
Subsequently it got modified a bit in 7da56142c, and then got completely
broken by bbdda2810 which designated _dbh_details() as an off-limits internal
method.

Given nothing has been heard since (6 years), and there is no effective change
of behavior (the snippet below fails identically below and after this change)
go ahead and burninate.

Also adjust the "can't touch this" exception text a bit.

DBICTEST_VIA_REPLICATED=1 perl -Ilib -It/lib -MDBICTest -e '
  eval { DBICTest->init_schema->storage->$_ }, warn $@
    for qw( _dbh_details _server_info _get_server_version _describe_connection )
'