global_data.h: drop write-only field dm_root_f

The dm_root_f field seems to be entirely write-only and hence
redundant, unless 'git grep' fails to find some access generated via
preprocessor token concatenation or similar.

Signed-off-by: Rasmus Villemoes <rasmus.villemoes@prevas.dk>
Reviewed-by: Simon Glass <sjg@chromium.org>
diff --git a/common/board_r.c b/common/board_r.c
index c823cd2..d4ba245 100644
--- a/common/board_r.c
+++ b/common/board_r.c
@@ -230,8 +230,7 @@
 
 	oftree_reset();
 
-	/* Save the pre-reloc driver model and start a new one */
-	gd->dm_root_f = gd->dm_root;
+	/* Drop the pre-reloc driver model and start a new one */
 	gd->dm_root = NULL;
 #ifdef CONFIG_TIMER
 	gd->timer = NULL;