Removing the unnecessary warning when default exception mapper is disabled

Discussed in https://github.com/resteasy/resteasy-microprofile/discussions/46

When the default exception mapper in MicroProfile Rest Client is disabled by setting the microprofile.rest.client.disable.default.mapper=true, a warning appears in the log for every single calls to RestClientBuilderImpl.build(). The log is added in https://github.com/resteasy/resteasy/pull/2850 and you can see that there is also a comment by reviewer about reducing the log level for this message to debug, but it got merged anyway.

I was wondering if it should really be considered as warning :thinking:

We have recently upgraded to WildFly 26 and there are tons of such warnings in the log while it's an expected behavior in our case. I think others could have had a similar issue too.