chore: remove un-needed tsc in demo scripts

[vakrilov]

PR Checklist

• The PR title follows our guidelines: https://github.com/NativeScript/NativeScript/blob/master/CONTRIBUTING.md#commit-messages.
• There is an issue for the bug/feature this PR is for. To avoid wasting your time, it's best to open a suggestion issue first and wait for approval before working on it.
• All existing tests are passing
• Tests for the changes are included

What is the new behavior?

Remove unneeded npm run tsc when starting demo projects. Webpack builds the typescript.

[tgpetrov]

npm run tsc actually also makes npm i, which step I believe is still important