design-template: remove Primary Assignee

Nowhere do we document what a "Primary Assignee" is and I don't know
what it means.  Drop it from the template.

Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
Change-Id: Ib107f90592965cf07982b5c576804938952fb256
diff --git a/designs/design-template.md b/designs/design-template.md
index 858c128..420df2c 100644
--- a/designs/design-template.md
+++ b/designs/design-template.md
@@ -30,9 +30,6 @@
 Author:
   < Name and Discord nickname >
 
-Primary assignee:
-  < Name and/or Discord nickname or `None` >
-
 Other contributors:
   < Name and/or Discord nickname or `None` >