well, most likely you just started the 30 day test period of LR Mobile with your registration of LR6.
Personally I dont see any use for this whole Mobile stuff, when I'm editing Images i feel more 'home' on my desktop PC with a calibrated Monitor and a glare free and consistant illumination.
for Your uninstall question, I think its save to remove. Your presets are all stored within your user profile, as far as I know nothing relevant (besides the programm itself) is in the LR5.x Install folder.
To be sure, rename the old LR 5.7 Program folder, work with lr6 for a while and then you may savely uninstall (rename the lr5.7 folder back before uninstalling)