If my previous guess was wrong, then it appears you must have selected "Draw this image service using tiles from cache" in the caching tab when you published it. Can you confirm?
This generates raster tile cache, which is the same as the imagery seen in ArcGIS base maps (as well as Google, Bing, etc.) - this is the fastest mode for serving imagery, but it's fast because the imagery is pre-rendered and the server simply looks up the tiles and transmits them. It is no longer a dynamic image service, so changing raster functions is no longer possible.
In either a map service or a cached image service, under the layer, there are 3 dots to pull down "more options", but none of the other modes that appear on a dynamic image service - e.g. see below - where you would have icons for "Show Legend", "Show Table", "Filter" and also the 3 dots for "more options" where you'll be able to select any server raster functions that are available.
I should verify - is the shaded relief working properly, and your concern is that you'd like to be able to disable it, or select other functions? If you need other functions, you'll need to republish as a dynamic image service (and you may want to find and delete the cache that had been generated). If all you will ever want is the shaded relief, the cached service should work for you.
Cody