wifi: brcmsmac: Remove unused brcms_debugfs_get_devdir()

brcms_debugfs_get_devdir() has been unused since it was added by commit
8e21df23 ("brcmsmac: hardware info in debugfs")

Remove it.

Signed-off-by: Dr. David Alan Gilbert <linux@treblig.org>
Signed-off-by: Kalle Valo <kvalo@kernel.org>
Link: https://patch.msgid.link/20241004004259.470853-1-linux@treblig.org