When compatibleOnly == false, the logic was equivalent to returning
singleOrNull.
The only call-site with compatibleOnly = true was FirElementSerializer,
but after the fix for KT-59613, the map will never contain multiple
entries when one of the entries is compatible.
Assuming that we only run serialization on green code, the map will
either contain one compatible entry or no entries which makes the
additional check redundant.
Kirill Rakhman
1d3a127f52