kbuild: rpm-pkg: avoid the warnings with dtb's listed twice

After 8d1001f7 (kbuild: rpm-pkg: fix build error with CONFIG_MODULES=n),
the following warning "warning: File listed twice: *.dtb" is appearing for
every dtb file that is included.
The reason is that the commented commit already adds the folder
/lib/modules/%{KERNELRELEASE} in kernel.list file so the folder
/lib/modules/%{KERNELRELEASE}/dtb is no longer necessary, just remove it.

Fixes: 8d1001f7

 ("kbuild: rpm-pkg: fix build error with CONFIG_MODULES=n")
Signed-off-by: Jose Ignacio Tornos Martinez <jtornosm@redhat.com>
Reviewed-by: Nathan Chancellor <nathan@kernel.org>
Tested-by: Nathan Chancellor <nathan@kernel.org>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>