Source mapping should not log warnings when service.version is unspecified

[axw]

For RUM spans and errors, we will log a warning for stack traces if service.version is undefined and source mapping is enabled:

apm-server/model/stacktrace.go

Line 62 in acd41cc

logp.NewLogger(logs.Stacktrace).Warn(msgServiceInvalidForSourcemapping)

Source mapping is enabled by default when RUM is enabled. Source mapping isn't necessarily expected by users, so we should not be logging warnings.

Assuming we will move source mapping entirely out of APM Server (#3606) then we may as well remove the logging now.

[axw]

Some kind of notification in the UI would likely be more visible to users. That could be done in a similar way, by identifying services without version defined.