chore: remove unused include for sql_statement.h

Removed unused sql_statement.h include.

Author: diegomarquezp

google/cloud/bigtable/bound_query.h

@@ -17,7 +17,6 @@
 
 #include "google/cloud/bigtable/instance_resource.h"
 #include "google/cloud/bigtable/internal/query_plan.h"
-#include "google/cloud/bigtable/sql_statement.h"
 #include "google/cloud/bigtable/value.h"
 #include "google/cloud/bigtable/version.h"
 #include "google/cloud/completion_queue.h"