The tab-bar always shows even though tab-bar-show is nil.

To reproduce:

    emacs -Q
    M-x tab-bar-mode
    M-: (setq tab-bar-show nil)

In GNU Emacs 28.0.50 (build 1, x86_64-pc-linux-gnu, GTK+ Version 3.24.21, cairo version 1.17.3)
 of 2020-08-08 built on lilydjwg
Repository revision: 7196abecb5f5c3cc1282280d2d337b6a86761656
Repository branch: master
Windowing system distributor 'The X.Org Foundation', version 11.0.12008000
System Description: Arch Linux