Remove unnecessary function declaration

void adress_accumulato() (sic) was declared but unused. Addressing
the accumulator is just another form of "implicit addressing" and ASL,
LSR, ROL, and ROR, are the only instructions using ACC adressing, and
they don't have implicit addressing variants.

Solves #1

include/cpu.h

@@ -123,7 +123,6 @@ class CPU
     void addressing_indirect_indexed();
 
     void addressing_implicit();
-    void address_accumulator();
     void addressing_immediate();
     void addressing_zero_page();
     void addressing_absolute();