Is it necessary to report every single finding, even if they're not relevant? I conducted a study a while back that I'm writing a manuscript on for publication.

The study was broken down into 5 different parts. Let's call them parts A, B, C, D, and E for simplicity.

Suppose that parts A through D were all connected to each other, but part E only provided descriptive data from a set of items that hasn't even been tested for Cronbach's alpha. I included part E more or less out of sheer curiosity, but it presents no reliable data or relevant information to the understanding of parts A through D. It also isn't important in replicating the study, nor does it counter any findings in parts A through D. In other words, it's more of an extension to the first four parts, but not directly related to the findings of them.

Is it necessary to include part E in the manuscript (i.e., in the methods, results, and discussion sections) since parts A through E were administered in the same overall study?
Or is it okay to simply not report parts of the study, so long as it does not bias the results of the parts you are reporting?
You can consider including this in your "Supporting Information", which is available in many publications. Sometimes, for the sake of presenting a coherent and concise manuscript, it may not be necessary or could even be detrimental to add portions which are orthogonal or simply irrelevant to the main line of your argument. However, by including it in the supporting information, it can still provide the necessary exposure of these findings without disrupting the flow of the paper. It can also have the additional benefit of allowing reviewers to see additional work done or preempt certain questions they may have in their review.