```
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`.
util: dedup code in callers of serviceFlagToStr()
```
Backport of core PR19106.
Depends on D8518.