wifi: brcmfmac: remove unused brcmf_usb_image struct

struct brcmf_usb_image was added in the initial commit 71bb244ba2fd5
("brcm80211: fmac: add USB support for bcm43235/6/8 chipsets") and updated
in commit 803599d40418 ("brcmfmac: store usb fw images in local linked
list.")

Its only usage was removed in commit 52f98a57d8c1 ("brcmfmac: remove
firmware list from USB driver").

Remove the structure definition now. This saves a few lines of code.

Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Acked-by: Arend van Spriel <arend.vanspriel@broadcom.com>
Signed-off-by: Kalle Valo <kvalo@kernel.org>
Link: https://msgid.link/23afd8c1733ad087ce2399a07a30d689aef861d5.1714039373.git.christophe.jaillet@wanadoo.fr