b43: Remove empty function lpphy_papd_cal()

In commit d825db346270e ("b43: shut up clang -Wuninitialized variable
warning"), the message noted that function lpphy_papd_cal() was empty
and had an old TODO regarding its implementation. As the reverse
engineering project that created the LP-PHY version of this driver
has not been active for some time, it is safe to remove this empty
function.

Signed-off-by: Larry Finger <Larry.Finger@lwfinger.net>
Cc: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>