Remove brave-core specific Welcome page feature

[petemill]

Changes

Removes Welcome from brave-ui since it has already been moved to brave-core via brave/brave-core#2896

Partially addresses brave/brave-browser#2335

Test plan

Storybook still works. Tests still work.

Link / storybook path to visual changes

• https://brave-ui-nva4w4sst.now.sh

Integration

Backwards compatible since brave-core no longer uses brave-ui for Welcome page!

• Does this contain changes to src/components or src/

  • Will you publish to npm immediately after this PR, or wait until sometime in the future?
  • Incompatible API change to something existing (major version increase)
  • Adding new backwards-compatible functionality? (minor version increase)
  • Fixing a bug backwards-compatibly? (patch version increase)

• Does this contain changes to src/features for brave-core?

  • Are there non backwards-compatible changes required for brave-core? Do not merge until brave-core PR is approvable. Link to brave-core PR:
  • Will you create brave-core PR to update to this commit after it is merged?
  • Wants uplift to brave-core feature branch?
    • When uplift-approved, merge to brave-core-0.VV.x feature branch
    • Create additional brave-core PRs for each feature branch to update commit

[cezaraugusto]

💯thanks Pete!

[cezaraugusto]

btw once we clean up the feature components we should start thinking about where our feature themes will be hosted.