Remove GRUB & Edit Boot Loader Records?
While it should be possible to either make an EFI/aware GRUB the primary boot loader that chainloads macOS and also the Mac startup manager to recognise Linux, there are some difficulties and downsides to either approach. Both weren't really made for each other and if FileVault2 comes into the picture things leave the pretty phase.

The currently nicest solution for dual booting macOS and Debian would have to be using The rEFInd Boot Manager. 


  rEFInd is a boot manager, meaning that it presents a menu of options
  to the user when the computer first starts up, as shown below. rEFInd
  is not a boot loader, which is a program that loads an OS kernel and
  hands off control to it. […] Many popular boot
  managers, such as the Grand Unified Bootloader (GRUB), are also boot
  loaders, which can blur the distinction in many users' minds.