[Cleanup] Use default ctor instead of an empty ctor. (#3206)

# Notes
- Use `= default;` instead of an empty ctor.
- https://pvs-studio.com/en/docs/warnings/v832/

common/eqdb.cpp

@@ -23,9 +23,6 @@
 
 EQDB EQDB::s_EQDB;
 
-EQDB::EQDB() {
-}
-
 unsigned int EQDB::field_count() {
 	return mysql_field_count(mysql_ref);
 }