Remove the header line after disabling 'which-function-mode'

Remove the header line after disabling 'which-function-mode'

Previously, the header line would stay around even when after
disabling 'which-function-mode', although it may be empty.  Now
the 'which-function-mode' element is properly removed from
'header-line-format', so the header line will disappear if
there's nothing else in 'header-line-format'.

Also, previously, when we ran (which-function-mode), we would
enable
'which-function-mode' for all buffers even if they didn't support
imenu.  We didn't run the normal logic in 'which-func-ff-hook' to
disable 'which-func-mode' if imenu wasn't present.  Now we do run
that logic, by just calling 'which-func-ff-hook'.  This is
especially important when the header line is enabled, because
otherwise there's a very noticeable header line added to every
buffer, including e.g. *Help* and *Buffer List*.

Also, we now check that 'header-line-format' is a list before trying
to add to it; this makes us work properly when enabling and
disabling 'which-function-mode' for modes which set
'header-line-format' to a string or symbol, such as eww.

* lisp/progmodes/which-func.el (which-func-try-to-enable): Re-add
'which-func-format' to the header line.
(which-func--header-line-remove): New function.
(which-func--disable): Call 'which-func--header-line-remove'.
(which-function-mode): Call 'which-func-ff-hook' and
'which-func--header-line-remove'. (bug#66283)
* test/lisp/progmodes/which-func-tests.el: New test.