disable warning from `drop table if exists`
The "warning" is in fact a "notice". You can disable the display of those notices trough the property client_min_messages  which controls the minimum level of messages returned to the client. 

To only display warnings, but not notices use:

set client_min_messages = warning;


Note that this will disable all notices, including those sent through raise notice from e.g. a PL/pgSQL function.