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/ringct/rctSigs.cpp

@@ -725,7 +725,6 @@ namespace rct {
         CHECK_AND_ASSERT_THROW_MES((kLRki && mscout) || (!kLRki && !mscout), "Only one of kLRki/mscout is present");
         keyV tmp(rows + 1);
         keyV sk(rows + 1);
-        size_t i;
         keyM M(cols, tmp);
 
         keyV P, C, C_nonzero;
@@ -899,7 +898,6 @@ namespace rct {
             key R;
             geDsmp P_precomp;
             geDsmp C_precomp;
-            geDsmp H_precomp;
             size_t i = 0;
             ge_p3 hash8_p3;
             geDsmp hash_precomp;