Kconfig: net: Remove deprecated `NET_BUF_USER_DATA_LEN`

This Kconfig setting was deprecated since
commit 8e99db580126 ("Kconfig: net: deprecate `NET_BUF_USER_DATA_LEN`")
and zephyr release 3.0.0.

This commit removes this deprecated and unused setting.

Signed-off-by: Christian Taedcke <christian.taedcke@lemonbeat.com>
diff --git a/subsys/net/Kconfig b/subsys/net/Kconfig
index 0018075..a8b8f94 100644
--- a/subsys/net/Kconfig
+++ b/subsys/net/Kconfig
@@ -13,12 +13,6 @@
-	int "Size of user_data available in every network buffer (DEPRECATED)"
-	default 0
-	help
-	  User data size is now set on a per-pool basis at declaration point
 config NET_BUF_LOG
 	bool "Network buffer logging"
 	select LOG