Removed grub and shim packages because they conflict with ubiquity

Ubiquity uninstalls packages related to other architectures after installing
grub (i.e. grub-efi-amd64-signed on BIOS systems). Having any of the grub or
shim packages in the keep-installed preseed variable lets ubiquity abort the
installation process with an error message.

packages

@@ -88,8 +88,6 @@ LD   |vim|
 LD   |htop|
 LD   |git|
  D   |gparted|
- D   |grub-efi-amd64-signed|
- D   |shim-signed|
 
 #texlive
  D   |dvidvi|