syslinux: remove host variant

The host variant of syslinux wasn't exposed anywhere, so the only way
to use it was to know that a "host-syslinux" target existed. Moreover,
thanks to commit 8e0d4118 ('syslinux:
install helper programs to the host'), the host utilities of syslinux
are automatically installed to $(HOST_DIR) when building the target
syslinux.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>