Simplify pseudo key

[minad]

This is something I noted while looking into the menu-item issue, https://github.com/justbur/emacs-which-key/issues/177#issuecomment-745521379. I propose to simplify the which-key--pseudo-key.

Advantages:

• The code gets a little bit simpler
• No unnecessary generation of symbols
• I expect lookup to get marginally faster
• The describe-bindings view looks slightly less cluttered to my eyes

(Copyright assigment - not needed since less than 15 lines)

[minad]

@justbur Thank you!

[justbur]

Thank you! I simplified a little further