stages/io.weldr.grub2: drop unused functions and options

Since we no longer use grub2-mkconfig, but write static configuration
we can drop most of the helpers.

The partitin table id was never used in the first place. We use
filesystem UUIDs, not partition UUIDs to name our root/boot partitions.

Signed-off-by: Tom Gundersen <teg@jklm.no>

samples/base-qcow2.json

@@ -1,6 +1,6 @@
 {
   "name": "base-qcow2",
-  "base": "445c09ba71fd656aadcc2c1e84adb12dcc5fd193fcbdc386b63e2fc02e134e98",
+  "base": "d7f6aca0b9cf09833f70983c20ef3ac6e626152ee99901fc07f26cbf85c2bced",
   "assembler":
     {
       "name": "io.weldr.qcow2",

samples/base-with-grub2.json

@@ -6,8 +6,7 @@
       "name": "io.weldr.grub2",
       "systemResourcesFromEtc": ["/etc/grub.d"],
       "options": {
-        "root_fs_uuid": "76a22bf4-f153-4541-b6c7-0332c0dfaeac",
-	"partition_table_id": "0xdeadbeef"
+        "root_fs_uuid": "76a22bf4-f153-4541-b6c7-0332c0dfaeac"
       }
     }
   ]