clickhouse: don't SET merge_tree_allow_nullable_key

Setting it was unnecessary and did not help.

The previous change #172
was motivated by an incorrect diagnosis of the problem:

This flag is a merge tree setting and not a global setting, so it's
part of the table definition and does not need to be set on the
session.

The real problem was ClickHouse/ClickHouse#54814
which was introduced in ClickHouse 23.7 and fixed in ClickHouse >23.9
and is specific to projections inside a table:

ClickHouse/ClickHouse@781674b
ClickHouse/ClickHouse@973cd5e

Another problem is that checkProperties (or checkKeyExpression) is not
called when doing: `ALTER TABLE ... MODIFY SETTING
allow_nullable_key=false` on a table that relied on a nullable key,
but that requires an upstream fix.

astacus/coordinator/plugins/clickhouse/steps.py

@@ -516,7 +516,6 @@ def _create_dbs(client: ClickHouseClient) -> Iterator[Awaitable[None]]:
             b"SET allow_experimental_object_type=true",
             b"SET allow_suspicious_codecs=true",
             b"SET allow_suspicious_low_cardinality_types=true",
-            b"SET merge_tree_allow_nullable_key=true",
             # If a table was created with flatten_nested=0, we must be careful to not re-create the
             # table with flatten_nested=1, since this would recreate the table with a different schema.
             # If a table was created with flatten_nested=1, the query in system.tables.create_table_query

tests/integration/coordinator/plugins/clickhouse/test_plugin.py

@@ -147,6 +147,7 @@ async def setup_cluster_content(clients: List[HttpClickHouseClient], use_named_c
         b"SETTINGS allow_experimental_object_type=1"
     )
     # test that we can re-create a table requiring custom merge tree settings.
+    # Unlike other cases, we don't have to SET a global settings because it's merge tree setting.
     await clients[0].execute(
         b"CREATE TABLE default.with_nullable_key (thekey Nullable(UInt32), thedata String) "
         b"ENGINE = ReplicatedMergeTree ORDER BY (thekey) "

tests/unit/coordinator/plugins/clickhouse/test_steps.py

@@ -788,7 +788,6 @@ async def test_creates_all_replicated_databases_and_tables_in_manifest() -> None
         b"SET allow_experimental_object_type=true",
         b"SET allow_suspicious_codecs=true",
         b"SET allow_suspicious_low_cardinality_types=true",
-        b"SET merge_tree_allow_nullable_key=true",
         b"SET flatten_nested=0",
         b"CREATE TABLE db-one.table-uno ...",
         b"CREATE TABLE db-one.table-dos ...",
@@ -847,7 +846,6 @@ async def test_creates_all_replicated_databases_and_tables_in_manifest_with_cust
         b"SET allow_experimental_object_type=true",
         b"SET allow_suspicious_codecs=true",
         b"SET allow_suspicious_low_cardinality_types=true",
-        b"SET merge_tree_allow_nullable_key=true",
         b"SET flatten_nested=0",
         b"CREATE TABLE db-one.table-uno ...",
         b"CREATE TABLE db-one.table-dos ...",