Remove rpl_semi_sync_[master,slave] plugin keyword usage in code

show variables like 'rpl_semi_sync_slave%';

Variable_name	Value

rpl_semi_sync_slave_delay_master	OFF

rpl_semi_sync_slave_enabled	OFF

rpl_semi_sync_slave_kill_conn_timeout	5

rpl_semi_sync_slave_trace_level	32

inline bool is_semi_sync_slave()

{

  int null_value;

  long long val= 0;

  get_user_var_int("rpl_semi_sync_slave", &val, &null_value);

  return val;

}