I don't know about that machine specifically, since I've never even seen an L2550.
The pumps I've seen or messed with required pulling a cover and then removing the linkage for the governor, removing the mounting bolts, then clocking the pump, fiddling with the rack and finally sliding it out.
How are you able to start the tractor? Once it's going, does it work fine at full throttle? Is it just when throttling down that it wants to die? I'd look closely at everything going on there before trying to pull the pump off. There's not anything really user-serviceable in there anyway.
Investigate the shutoff linkage and throttle linkage. It's possible something is amiss or broken there.
Do I understand you correctly that if you pull the injector lines off and crank the engine no fuel comes out? If so, try gently tapping on the pump with a hammer as you crank the engine, to see if you can free a stuck plunger.
I am still not totally clear on the symptoms you're experiencing. I've put some questions all together, though some are repeats. Hopefully others with more knowledge than me will chime in, too:
1. Does the tractor run ok once started? Normal power, rpm, etc?
2. What color, if any, smoke do you see when running and starting up?
3. Will it run at part throttle, and/or idle?
4. When you say it wants to die when the throttle is reduced, what happens if you go from full throttle to half throttle?
5. What happens at the injector lines when they are removed and you crank the engine, with the throttle set at full?
6. Is the reduced throttle shut-off of the engine intermittent, or consistent?
Good luck, you'll get it figured out. Post back with what information you can so we can help.