fix build with -fno-elide-constructors

Linker is trying to find a copy constructor to
injector::transaction::transaction

ld.lld: error: undefined symbol: injector::transaction::transaction
(injector::transaction const&)
>>> referenced by rpl_injector.cc:164
>>>               rpl_injector.cc.o:(injector::new_trans(THD*))

This constructor is declared, but is not implemented. Ok if copy elision
is enabled, but causes error otherwise.

Remove the constructor declaration, so that operator= will be used in
reality.

sql/rpl_injector.h

@@ -146,7 +146,6 @@ public:
       };
 
       transaction() : m_thd(NULL) { }
-      transaction(transaction const&);
       ~transaction();
 
       /* Clear transaction, i.e., make calls to 'good()' return false. */