SPYT-1033: revert cluster-default network_project from SPYT gateway

The cluster-level NetworkProject default added in the previous PR was
conceptually wrong: spark.ytsaurus.network.project is a property of
each user's ABC access, not of the SPYT cluster. There is no universal
default that works for all users, so any value baked into gateway
config either misleads or breaks queries.

The correct path is per-query override via the existing
parameters.spark_conf.spark.ytsaurus.network.project hook in
SpytOperationsRunner.formatSettings; that path is unaffected by this
revert.

Reverts the following components added in PR 14020425:
- TSpytClusterConfig.NetworkProject proto field
- Cluster.CustomFieldType.SPYT_NETWORK_PROJECT enum value
- 6th Optional<String> networkProject parameter of Cluster.newSpyt
- networkProject reading in ClustersManager.getSpytClusters
- SpytOperationsRunner.buildSparkConfFromCluster and the third
  cluster layer in the spark_conf merge
- SPARK_NETWORK_PROJECT_KEY constant
- NetworkProject: "test_spyt_project" line in yql-test/gateways.conf
- Related tests + the spytCluster(discoveryPath, networkProject)
  helper overload

DiscoveryPath stays — it is a legitimate cluster property (canonical
discovery path set by ops once per cluster) and is unrelated to
per-user ABC access.
commit_hash:9c167a5ebef7c3458f1691fe00c81674bb694789

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