You should be able to do that, yes. The nginx repo installs what it needs on its own and I'd wager those packages aren't necessary.
See what happens when you run "apt-get remove nginx-common". Most likely only those two packages will be removed, leaving nginx-1.8.1 installed.
See what happens when you run "apt-get remove nginx-common". Most likely only those two packages will be removed, leaving nginx-1.8.1 installed.