Remove binary_log_enabled from Postgres test, as it's unsupported (hashicorp#2075)

\+ update docs.

This must have silently failed in the API before? It doesn't work & is tested upstream now. Binary logs are a non-feature of Postgres.

Author: rileykarson

docs/r/sql_database_instance.html.markdown

@@ -190,7 +190,7 @@ The optional `settings.database_flags` sublist supports:
 The optional `settings.backup_configuration` subblock supports:
 
 * `binary_log_enabled` - (Optional) True if binary logging is enabled. If
-    `logging` is false, this must be as well.
+    `logging` is false, this must be as well. Cannot be used with Postgres.
 
 * `enabled` - (Optional) True if backup configuration is enabled.