Remove "Conflicts: libmariadbclient18 (<< 10.2.0)" (Closes: #917075)

When upgrading from a system with libmariadbclient18 installed, this
conflict line forces libmariadbclient18 to be uninstalled when in fact
it should only be upgraded.

The line was imported from upstream sources where their origin in a bit
unclear and probably don't have any function in Debian official.