ncurses: get rid of unused BR2_PACKAGE_NCURSES_TARGET_HEADERS handling

The option was removed from Config.in in 58508f39 (ncurses: remove
dedicated target-headers option), so remove it from the makefile as
well.

Reported-by: Ian <reg-ian.ridley-buildroot@hydrix.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>