Consider dropping base-browser branches

I am writing documentation about our branchs and rebase process and I realized I don't fully understand why we need base-browser branches.

The tags serve the purpose of marking the end end of base-browser commits and during my workflow I do not remember ever using base-browser branches for anything other than cherry picking.

Which brings me to my next point, that if we drop this branch it's one less thing to do whenever we merge base-browser commits 😅 W would only need to cherry-pick to mullvad-browser.

What do you all think?

@dan @clairehurst @pierov @henry @boklm @morgan @ma1