boards: xenvm: remove incorrect condition for Kconfig heap values

xenvm Kconfig contained incorrect name for board parameter. It led to
build issues - heap size was set incorrectly. Since whole file is
already placed under right Kconfig condition ("if BOARD_XENVM"), remove
incorrect parameter at all.

This issue was introduced by commit 8dc3f856 ("hwmv2: Introduce
Hardware model version 2 and convert devices") due to the typo.

Signed-off-by: Dmytro Firsov <dmytro_firsov@epam.com>