nilrt.inc: Remove GRUB_BRANCH variable
nilrt.inc: Remove GRUB_BRANCH variable

The GRUB_BRANCH variable was only used to generate ni_grub_version
written to the grub-ni-version file. Since the version is not used
at this time, remove the GRUB_BRANCH variable.

Signed-off-by: Shruthi Ravichandran <db26f49c821f5e8eba3f4f30a1d200e2416b0696@ni.com>

ni/meta-nilrt,ni/meta-nilrt,ni/meta-nilrt,ni/meta-nilrt,ni/meta-nilrt