Hardcode disable option as it is not valid for postgres

[AkhilGNair]

This removes backup_configuration.binary_log_enabled as an option in the postgres module, as attempting to set it as true results in an error:

Error: Error, failed to create instance <instance-name>: googleapi: Error 400: Invalid request: Binary log can only be enabled for MySQL instances., invalid

  on .terraform/modules/terraform-google-sql/modules/postgresql/main.tf line 26, in resource "google_sql_database_instance" "default":
  26: resource "google_sql_database_instance" "default" {