How to delete replication slot in postgres 9.4

As a complement to the accepted answer, I'd like to mention that following command will not fail in case the slot does not exist (this was useful for me because I scripted that).

select pg_drop_replication_slot(slot_name) from pg_replication_slots where slot_name = 'bottledwater';

Use pg_drop_replication_slot:

select pg_drop_replication_slot('bottledwater');

See the docs and this blog.

The replication slot must be inactive, i.e. no active connections. So if there's a streaming replica using the slot you must stop the streaming replica. Or you can change its recovery.conf so it doesn't use a slot anymore and restart it.