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).

Zcash: cherry-picked from commit 7650449a6777710cf818d41862626164da0cd412
Dropped changes to qa/rpc-tests/rest.py pending addition of /rest/headers/

src/rpcserver.cpp

@@ -123,7 +123,7 @@ void RPCTypeCheckObj(const UniValue& o,
 
 CAmount AmountFromValue(const UniValue& value)
 {
-    if (!value.isReal() && !value.isNum())
+    if (!value.isNum())
         throw JSONRPCError(RPC_TYPE_ERROR, "Amount is not a number");
     CAmount amount;
     if (!ParseFixedPoint(value.getValStr(), 8, &amount))