signal/sparc/restore_fpu_state: remove

A function never called from anywhere, obviously half-complete.
Remove function and if someone wants to complete this, please
check the old version out of git history.

Signed-off-by: Riku Voipio <riku.voipio@linaro.org>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
