Bluetooth: userchan: Remove support for native_posix and clarify msg

native_posix has been removed, so let's remove its support from
kconfig for this driver.
Also let's replace its mention in the help message.

Signed-off-by: Alberto Escolar Piedras <alberto.escolar.piedras@nordicsemi.no>
diff --git a/drivers/bluetooth/hci/Kconfig b/drivers/bluetooth/hci/Kconfig
index 186f89a..9556e8c 100644
--- a/drivers/bluetooth/hci/Kconfig
+++ b/drivers/bluetooth/hci/Kconfig
@@ -180,14 +180,14 @@
 
 config BT_USERCHAN
 	bool
-	depends on (BOARD_NATIVE_POSIX || BOARD_NATIVE_SIM)
+	depends on BOARD_NATIVE_SIM
 	default y
 	depends on DT_HAS_ZEPHYR_BT_HCI_USERCHAN_ENABLED
 	select NATIVE_USE_NSI_ERRNO
 	help
 	  This driver provides access to the local Linux host's Bluetooth
 	  adapter using a User Channel HCI socket to the Linux kernel. It
-	  is only intended to be used with the native POSIX build of Zephyr.
+	  is only intended to be used with the native_sim[//64] build of Zephyr.
 	  The Bluetooth adapter must be powered off in order for Zephyr to
 	  be able to use it.