Remove `@nightwatch/mobile-helper` from direct dependencies

[garg3133]

As @nightwatch/mobile-helper is quite bulky, it takes a lot of time for npm init nightwatch command to download create-nightwatch package and run it. So, we should consider removing it from the direct dependencies.

Since we anyway install @nightwatch/mobile-helper as a separate package in case user is interested in mobile testing, we can use that only while doing mobile setup in create-nightwatch.