chore: schema cleanup (#1522)

The columns referred to in this PR, were for some reason created with
defaults. When created years ago, they were `bigserial` not `bigint`.

The defaults were never actually used, as verified by

1. Checking the value of the sequences on the database
2. Checking the code

So we're safe to clean them up.

atuin-server-postgres/migrations/20240108124837_drop-some-defaults.sql

@@ -0,0 +1,4 @@
+-- Add migration script here
+alter table history alter column user_id drop default;
+alter table sessions alter column user_id drop default;
+alter table total_history_count_user alter column user_id drop default;