Remove unnecessary kernel post-install hook for GRUB-based targets (#3672)

The code for this hook was removed in #3457 but it wasn't removed from the
manifest. Remove it to avoid unnecessary execution of the hook.

buildroot-external/ota/manifest.raucm.gtpl

@@ -19,8 +19,7 @@ hooks=install;
 
 [image.kernel]
 filename=kernel.img
-{{- $bootloader := (env "BOOTLOADER") }}
-{{- if or (eq $bootloader "grub") (eq $bootloader "tryboot") }}
+{{- if eq (env "BOOTLOADER") "tryboot" }}
 hooks=post-install;
 {{- end }}