ringct: remove dead code

- h2b, b2h (rctTypes) and sumKeys (rctOps): from the original RingCT
  import (9b1afe5f2, 2016-05-13); never referenced by any caller since
  the import.
- cn_fast_hash128, hash_to_scalar128 (rctOps): from the same import.
  hash_to_scalar128 (and cn_fast_hash128 through it) was last used by
  the original MLSAG gen/ver hashing, which dbb5f2d6a (2016-07-09)
  replaced with keyV-based hash_to_scalar; dead since then.
- bos_coster_heap_conv (multiexp): added in 9ff6e6a0a (2018-01-09)
  alongside bos_coster_heap_conv_robust; multiexp() only ever
  dispatched the robust variant, so the non-robust one was never
  called. The robust variant is kept, as tests/performance_tests
  still uses it (production dispatch moved to Straus/Pippenger in
  939bc22/263431c).

src/ringct/rctOps.cpp

@@ -633,19 +633,6 @@ namespace rct {
         return hash;
      }
     
-    //cn_fast_hash for a 128 byte unsigned char
-    key cn_fast_hash128(const void * in) {
-        key hash;
-        keccak((const uint8_t *)in, 128, hash.bytes, 32);
-        return hash;
-    }
-    
-    key hash_to_scalar128(const void * in) {
-        key hash = cn_fast_hash128(in);
-        sc_reduce32(hash.bytes);
-        return hash;
-    }
-    
     //cn_fast_hash for multisig purpose
     //This takes the outputs and commitments
     //and hashes them into a 32 byte sized key
@@ -703,15 +690,6 @@ namespace rct {
       ge_p1p1_to_p3(&hash8_p3, &hash8_p1p1);
     }
 
-    //sums a vector of curve points (for scalars use sc_add)
-    void sumKeys(key & Csum, const keyV &  Cis) {
-        identity(Csum);
-        size_t i = 0;
-        for (i = 0; i < Cis.size(); i++) {
-            addKeys(Csum, Csum, Cis[i]);
-        }
-    }
-
     //Elliptic Curve Diffie-Hellman: encodes and decodes the amount b and mask a
     // where C= aG + bH
     key genAmountEncodingFactor(const key &k)