Unless I am wrong, the earliest versions of Rolemaster (or maybe it was M.E.R.P.?) had only Channeling and Essence. The Mentalism realm was introduced later. The idea of going back to only Channeling and Essence has crossed my mind several times, since, and this is just my personal opinion of course, Mentalism is by far the least interesting realm, and the contrast between divine magic and elementalism makes for a more flavorful magic system than a 'trinity' magic system without much interaction between the realms.
I can see some problems with removing a realm from the modern least outdated versions of RM, such as Mentalism professions having Pr as prime stat (would have to be changed if the professions are still used), Mentalism professions can wear armor, and have a low cost in armor skills - that would seem odd if they were changed to another realm, so maybe those costs should change, and they would then have to be compensated somehow.
A 'clean' way to remove the realm could be to remove all Mentalism professions as well - then you have a system with less options, but more consistensy.
The Pr stat would be even less useful than it is now.
Special abilites, spell descriptions, items, etc. that mention Mentalism would have to be changed or ignored.
It would probably require some work no matter how you do it, but it could work, I think.