staging: wlan-ng: remove unused function prototypes

staging: wlan-ng: remove unused function prototypes

These functions are declared but not defined or used anywhere. Their
definitions were removed 15 years ago.

prism2mgmt_set_grpaddr() was removed in the
commit 1e7201836c57 ("Staging: wlan-ng: Delete a pile of unused mibs.
And fix WEXT SET_TXPOWER.")
It's signature was changed in the commit 5a2214e2e02f ("staging:
wlang-ng: avoid new typedef: hfa384x_t")

prism2mgmt_get_grpaddr() and prism2mgmt_get_grpaddr_index() were
removed in the commit cbec30c4c00c ("Staging: wlan-ng: Delete a large pile of
now-unused code.")

Signed-off-by: Calvince Otieno <calvncce@gmail.com>
Link: https://lore.kernel.org/r/ZSxFS8EnFr998PqS@lab-ubuntu
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>