kernel-install: don't fail on kernel removal in non-EFI installs

In non-EFI installation /boot/efi/EFI/qubes may not exists. In this case
do not try to touch (non-existing) files there.

Fixes QubesOS/qubes-issues#1829

system-config/kernel-xen-efi.install

@@ -64,6 +64,7 @@ case "$COMMAND" in
     remove)
         # don't care about changing default= line - yum should prevent removing
         # currently running kernel
+        if [ -r $EFI_DIR/xen.cfg ]; then
             awk -F = --assign "kver=${KVER}" '
               /^\[/ {
                   # section header - previous section (if any) ended
@@ -80,6 +81,7 @@ case "$COMMAND" in
                   }
               }' $EFI_DIR/xen.cfg > $EFI_DIR/xen.cfg.new
             mv $EFI_DIR/xen.cfg.new $EFI_DIR/xen.cfg
+        fi
         rm -f "$EFI_DIR/initramfs-${KVER}.img"
         ;;
 esac