univalue: Avoid unnecessary roundtrip through double for numbers

JSON makes no distinction between numbers and reals, and our code
doesn't need to do so either.

This removes VREAL, as well as its specific post-processing in
`UniValue::write`. Non-monetary amounts do not need to be forcibly
formatted with 8 decimals, so the extra roundtrip was unnecessary
(and potentially loses precision).

src/univalue/univalue_write.cpp

@@ -61,13 +61,6 @@ string UniValue::write(unsigned int prettyIndent,
     case VSTR:
         s += "\"" + json_escape(val) + "\"";
         break;
-    case VREAL:
-        {
-            std::stringstream ss;
-            ss << std::showpoint << std::fixed << std::setprecision(8) << get_real();
-            s += ss.str();
-        }
-        break;
     case VNUM:
         s += val;
         break;