[FIXED JENKINS-24176] Avoid loading builds from disk just to display test result trend

Fixing [JENKINS-24176](https://issues.jenkins-ci.org/browse/JENKINS-24176) by:

* Making a fix similar to that of JENKINS-23945: stop looking backwards in trend graph when we seem to come to an unloaded build.
* Reverting fix of JENKINS-9839, since it is too expensive (as of Jenkins 1.485) to be worthwhile.
> Does this mean that if the previous build was deleted, the graph won't be displayed?

Well, the left side of the graph will be truncated at the deleted build, rather than skipping over it to earlier remaining builds as the original code would have done.

In practice this situation is not common, since the normal reason for builds to be deleted is that `LogRotator` has run on them, which always works in chronological order; i.e., typically the set of existing builds will be a contiguous range.