chore: Remove old Vanilla 2.0 related workarounds

[bartaz]

Done

Cleaning old CSS workarounds (mostly around Vanilla 2.0 references):

• removed custom p-logo-link styles (from unused first snap flow pages)
• removed unused u-flex utility and all its outdated use in publisher header (utility itself has been replaced by u-flex-column in other places)
• removed unnecessary workaround for code snippet issue
• removed unnecessary z-index from release UI use of contextual menu

How to QA

• Open demo

• Go to first snap flow: https://snapcraft-io-5012.demos.haus/first-snap/python

• Make sure Linux and MacOS cards are clickable

• Sign in, go to any snap Release UI, make sure contextual menu under Promote/Close still looks and works as expected:

• Go to Publicise tab of any snap, make sure Markdown code snippets wrap the code

Testing

• This PR has tests
• No testing required (explain why): just visual changes

Issue / Card

Fixes WD-18868

[webteam-app]

Demo

Jenkins

demos.haus

[bartaz]

A bit quick'n'dirty way to make this not look broken without removed p-logo-link.

These pages don't seem linked from anywhere anymore, and are supposed to be removed as part of WD-18896 anyway.

[steverydz]

LGTM 👍