grubcfg: Get the distribution name from the branding.

Remove the redundant "distributor" setting from grubcfg.conf (and the
entire grubcfg.conf file that has no settings left for now).

src/modules/grubcfg/grubcfg.conf

@@ -1,3 +0,0 @@
----
-# Replace 'LinuxDistribution' with your distribution name - for example with 'Manjaro'
-distributor: LinuxDistribution

src/modules/grubcfg/main.py

@@ -66,5 +66,6 @@ def modify_grub_default(partitions, root_mount_point, distributor):
 def run():
     partitions = libcalamares.globalstorage.value("partitions")
     root_mount_point = libcalamares.globalstorage.value("rootMountPoint")
-    distributor = libcalamares.job.configuration["distributor"]
+    branding = libcalamares.globalstorage.value("branding")
+    distributor = branding["productName"]
     return modify_grub_default(partitions, root_mount_point, distributor)