Converting full chords to power chords in midi

The usual power chord is usually just the root and the fifth.
 
So if you've got a full Amajor chord, for example, delete everything but the lowest A and the E above it.  I'd be surprised if there was an automated way to do this.

I'm afaid your wish is next to impossible. There's no way a robot can determine the chord viocing and only strip away the 3rd, 4t etc. The 4TH of a G chord is the root of a C chord so how on earth will that work. 
You need to play your own parts that way you want them ( the easy way) or suck it up and spand a lot of time editing. 

 "I'd imagine you could do a standard rock/pop song in 5-10 minutes."  
Optimistic timeline :) It would take me a lot longer, and I guess this all depends on the OP's knowledge of chord theory. 
 
A lot would depend on the complexity of the arrangement, A  3 chord song is way more possible than a 6 chord song. 
Example 3 chords G-C-D. 
G chord G B D   so B has to go
C chord C E G    so E has to go
D chord D F# A  so F# has to go   
So we are keeping G, D,  C and  A  
So you could do it in a couple of clicks using the Piano keys on the Left to highlight all the B, E and F#. at once. 
But then adding any other chords to the arrangement will probably require those notes again so now your back to editing chord by chord. 
 
And visually one would hope to just delete the middle note, but many of these are using inversions so you have to be careful.  
 
I think what the OP is dealing with are most free midi downloads are done by keyboard players who tend to not approach chords the way a guitar does..Then or worse, generated by Band in a Box type software  and they use overly complex chords which sound real bad when what was required was more fundamental tones.  It's rare that I can find a midi file that is well done. I was even just looking for a few Christmas songs and it blows me away how many bad versions there are of even a basic melody. I always end up rolling my own. It's much faster than polishing the old turdd 

Well thanks for the replies.  I was just wondering if there are chord detectors (which I think I've seen a few) there should be a way to generate a power chord replacement.  It's probably out there somewhere.  I tried lots of searches and came up empty.
 
I understand chord theory ok but my main problem is identifying the chords and then picking the notes to erase.  Like I said, I have done it manually for quite a while but it's a serious pain.