util: simplify the interface of serviceFlagToStr()

Don't take two redundant arguments in `serviceFlagToStr()`.

As a side effect this fixes an issue introduced in
https://github.com/bitcoin/bitcoin/pull/18165 due to which the GUI could
print something like `UNKNOWN[1033] & UNKNOWN[1033] & UNKNOWN[2^10]`
instead of `NETWORK & WITNESS`.

src/protocol.h

@@ -288,7 +288,12 @@ enum ServiceFlags : uint64_t {
     // BIP process.
 };
 
-std::string serviceFlagToStr(uint64_t mask, int bit);
+/**
+ * Convert a service flag (NODE_*) to a human readable string.
+ * It supports unknown service flags which will be returned as "UNKNOWN[...]".
+ * @param[in] bit the service flag is calculated as (1 << bit)
+ */
+std::string serviceFlagToStr(size_t bit);
 
 /**
  * Gets the set of service flags which are "desirable" for a given peer.