Remove unused variables in monero codebase

There are quite a few variables in the code that are no longer
(or perhaps never were) in use. These were discovered by enabling
compiler warnings for unused variables and cleaning them up.

In most cases where the unused variables were the result
of a function call the call was left but the variable
assignment removed, unless it was obvious that it was
a simple getter with no side effects.

src/wallet/ringdb.cpp

@@ -42,9 +42,6 @@
 
 #define V1TAG ((uint64_t)798237759845202)
 
-static const char zerokey[8] = {0};
-static const MDB_val zerokeyval = { sizeof(zerokey), (void *)zerokey };
-
 static int compare_hash32(const MDB_val *a, const MDB_val *b)
 {
   uint32_t *va = (uint32_t*) a->mv_data;