[rv_dm] Remove DV plan

The file is empty and should exist now in the doc folder.
diff --git a/hw/ip/rv_dm/dv/plan.md b/hw/ip/rv_dm/dv/plan.md
deleted file mode 100644
index 6362634..0000000
--- a/hw/ip/rv_dm/dv/plan.md
+++ /dev/null
@@ -1 +0,0 @@
-{{% lowrisc-doc-hdr RV_DM DV Plan }}