chore: Make image field obsolete in Website Item (redundant)

- Delete Image field and set `website_image` as form's image field for uploads
- Remove instances of `image` field access via Website Item
- Item -> Web Item via Desk: Map Item's `image` to Web Item's `website_image`
- Item -> Web Item via patch: `website_image` will be mapped with thumbnail
- Remove magic that auto-sets `website_image` from `image` in Website Item

(cherry picked from commit 9541354ec7)

# Conflicts:
#	erpnext/e_commerce/doctype/website_item/website_item.py

erpnext/patches/v13_0/create_website_items.py

@@ -17,7 +17,6 @@ def execute():
 		"item_group",
 		"stock_uom",
 		"brand",
-		"image",
 		"has_variants",
 		"variant_of",
 		"description",
@@ -30,6 +29,7 @@ def execute():
 		"website_warehouse",
 		"web_long_description",
 		"website_content",
+		"website_image",
 		"thumbnail",
 	]