tests: do not reserve disk space in full_node_test

Additional reservation of disk space influence balancer logic and makes
external calculation (based on actual disk space) inaccurate. Dropped
reservation in `full_node_test` as it is not requires since there are
no topics with reclaimable retention.

Fixes: redpanda-data#8220

Signed-off-by: Michal Maslanka <michal@redpanda.com>
(cherry picked from commit 70c0f7a)

tests/rptest/tests/partition_balancer_test.py

@@ -658,9 +658,8 @@ def test_full_nodes(self):
                 "health_monitor_max_metadata_age": 3000,
                 "log_segment_size": 104857600,  # 100 MiB
                 "retention_local_target_capacity_percent": 100.0,
-                # add disk reservation to buffer
-                "disk_reservation_percent": 10.0,
-                "retention_local_trim_interval": 3000
+                "retention_local_trim_interval": 3000,
+                "disk_reservation_percent": 0.0
             },
             environment={"__REDPANDA_TEST_DISK_SIZE": disk_size})