Kconfig: net: Remove deprecated `NET_BUF_USER_DATA_LEN`

This Kconfig setting was deprecated since
commit 8e99db58 ("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>