core/pkg-kernel-module: drop now-useless check for CONFIG_MODULES

This reverts commit 8df95d92 (pkg-kernel-module: die if kernel module
support is disabled).

Now that we force-enable support for modules in the kernel config, we
need not check it.

Besides, the check was broken, because it did not use $$ to dereference
LINUX_DIR, thus leading to systematic build failures when a package
using the kernel-module infra was enabled.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Peter Korsgaard <jacmet@uclibc.org>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Cc: Noé Rubinstein <noe.rubinstein@gmail.com>
Cc: Jan Viktorin <viktorin@rehivetech.com>
Reviewed-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>