Cleanup sql_prepare.cc

sql/sql_prepare.cc:
  Cleanup mysql_test_* family of calls: now we don't send error message
  from mysql_stmt_prepare, so no need to support -1 return code 
  (meaning error is not sent to client) in these functions.
  Move unit->cleanup() to mysql_stmt_prepare() as it's done in most
  of the mysql_test_ functions, and is a no-op for those which don't
  call unit->prepare(). This should make fixing of Bug#10729 (cursors)
  easier.