Disable which-func-mode in erc-imenu buffers

Disable which-func-mode in erc-imenu buffers

* lisp/erc/erc-imenu.el (erc-imenu-setup): Move after module definition
so the variable `erc-imenu-mode' is defined.  Run teardown code when
module is deactivated.  Set `which-func-mode' to nil locally.
(erc-imenu-mode, erc-imenu-enable, erc-imenu-disable): Manage membership
of `erc-imenu--disable-which-func' in `which-function-mode-hook'.
(erc-imenu--disable-which-func): New function.  (Bug#74934)

(cherry picked from commit d1c670f0cbf9c66d59b24797f38203ea698813c3)