luci-mod-dsl: drop dependency on ltq-dsl-base

The `ltq-dsl-base` package is nonshared and thus not processed on phase 2
builders, causing this luci module to be unavailable in repos. Furthermore,
the `ltq-dsl-base` package provides no functionality the ui requires, so
simply drop the dependency.

Ref: https://forum.openwrt.org/t/x/153515
Signed-off-by: Jo-Philipp Wich <jo@mein.io>