That is, if you took two individuals who were following an identical training and nutrition protocol with the only difference being that one masturbated more frequently than the other, would there be any noticeable variance in their bottom line results.
If frequent masturbation causes behavioral changes in you (such as decreased motivation, focus or energy levels) which then negatively carries over to your bodybuilding efforts, that’s a different story and is a topic for a different discussion.
With that out of the way, the answer to this question seems to be fairly straightforward at this point:
Masturbation and ejaculation do not appear to have any significant long term positive or negative effect on blood testosterone levels.
For that reason, assuming the consistency of your training and nutrition remains unaffected, there is no good reason to suspect that masturbating less frequently will improve your rate of muscle growth and fat loss, or vice versa.