Regression tests: Remove places where we explicitly set citus.shard_count to 4

We used to run most of our regression tests with the default shard count. We recently reduced the shard count number from 32 to 4.

We have regression tests where we explicitly reduced the shard count to 4. Now that our default shard count is 4, we should delete places where we reduced the shard count to speed up the regression tests.

Two examples:

* multi_behavioral_analytics_create_table.sql: `SET citus.shard_count = 4;`
* multi_upser.sql: `SELECT master_create_worker_shards('upsert_test', '4', '2')`