package/liquid-dsp: drop support for fftw-double and fftw-long-double

package/liquid-dsp: drop support for fftw-double and fftw-long-double

liquid-dsp has the following code:

So, if it uses the external fftw library, it can only use the fftwf
variant, i.e the fftw-single variant. Otherwise, it uses its internal
FFT library.

There is no way for liquid-dsp to use fftw-double or fftw-long-double.

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>