Recently we had a building error if we enable the MOUSE_PS2_ALPS while
disable the MOUSE_PS2_TRACKPOINT, and was fixed by 49e6979e
("Input: psmouse - fix build error of multiple definition").

We could improve that fix by dropping all unneeded functions and
CONFIG_MOUSE_ guards from the header, it is safe to do that since
those functions are not directly called by psmouse-base.c anymore.
Signed-off-by: Hui Wang <hui.wang@canonical.com>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>