TeX-texinfo-mode comes from the AUCTeX package. It is defined in tex-info.el.

You can find that information yourself by the key sequence C-h f TeX-texinfo-mode. (Note that the key sequence C-h f is bound to the command describe-function.)

You can get back the old behavior by customizing the variable TeX-modes via M-x customize-option RET TeX-modes RET. You can also use the menu item
Options → Customize Emacs → Specific Option....

Toggle texinfo-mode from the TeX-modes list off.

General Method to Remove an Advice

The opt-out of the override of texinfo-mode via TeX-modes is possible in AUCTeX.

The more general approach to revert the effect of an (override) advice is advice-remove after the advising package has been loaded.

This is demonstrated for texinfo-mode. One would put the following code in ones info file:

(eval-after-load "tex-info"
  (lambda ()
    (advice-remove 'texinfo-mode #'TeX-texinfo-mode)))

But, you do not need to do that since you have the customization option TeX-modes.