package/fftw: remove as regular package

fftw is no longer a package: only fftw-single, fftw-double,
fftw-long-double and fftw-quad are. fftw.mk only contains some common
definitions that are used by the different fftw variants.

The old BR2_PACKAGE_FFTW_PRECISION_* options, which are no longer used
anywhere, are moved to Config.in.legacy.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Signed-off-by: Gwenhael Goavec-Merou <gwenhael.goavec-merou@trabucayre.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>