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/qt/guiutil.cpp

@@ -759,7 +759,7 @@ QString formatServicesStr(quint64 mask)
         uint64_t check = 1ull << i;
         if (mask & check)
         {
-            strList.append(QString::fromStdString(serviceFlagToStr(mask, i)));
+            strList.append(QString::fromStdString(serviceFlagToStr(i)));
         }
     }