Yes it can be deleted. IT was probably put if you did a "Upgrade c5" from within the Dashboard which downloads the file into that tmp dir. It also copies it into the /upgrades DIR which can add to your overall install size. It is best to manually upload the c5 core into /concrete everytime. If you do that, be sure to remove the extra line that gets added to your config file. When you open it, it should be obvious which line I am referring to.

So long answer short, you can delete everything in the files/tmp folder as well as the files/cache folder.

Yes, I did do an upgrade. Thanks for the info.