refactor(spanner): remove unnecessary interface class (#15371)

Author: scotthart

google/cloud/spanner/internal/partial_result_set_source.h

@@ -37,7 +37,7 @@ namespace cloud {
 namespace spanner_internal {
 GOOGLE_CLOUD_CPP_INLINE_NAMESPACE_BEGIN
 
-class PrecommitInterface {
+class PartialResultSourceInterface : public spanner::ResultSourceInterface {
  public:
   /**
    * A precommit token is included if the read-write transaction is on
@@ -51,9 +51,6 @@ class PrecommitInterface {
   }
 };
 
-class PartialResultSourceInterface : public spanner::ResultSourceInterface,
-                                     public PrecommitInterface {};
-
 /**
  * This class serves as a bridge between the gRPC `PartialResultSet` streaming
  * reader and the spanner `ResultSet`, and is used to iterate over the rows