Remove 2nd partition, reclaim space
It's easily done, but for safety you should always backup your data as there is a small chance that things can go wrong.  Check your Time Machine or equivalent is up to date, then open the Disk Utility application.

Select the disk from the list of mounted drives on the left that corresponds to your internal disk.  If you look at it's partitions you should see 2 that correspond (roughly, this is the size of the partition, not the filesystem, so don't expect the figures to be 100% exact) to your Mac and Linux areas.  To remove the linux one, it should be as simple as selecting it, and hitting delete.  This will leave a linux shaped hole and leave your Mac partition as it stands.  Next, you can expand the Mac area by dragging it's size in the reference diagram, or manually changing it's size by clicking on it and changing the value in the box.

When you click Apply (sorry, I am not at a Mac and cannot recall the exact wording) it will make the changes which could take some time depending on the size of the partitions.

Turn off any disk intensive applications including Time Machine for the duration of the operation, and note that you should not reboot or do anything that you may think likely to cause a crash etc (if you use any unstable programs) until it is complete.

EDIT: An encrypted MAC partition cannot be resized, this is a backup, recreate, and restore scenario.