removed unused field _frequency and associated code

[gullradriel]

• Remove an unused field appearing only in audio WFM under the Rec button
• Removed unused label
• Resized WaveForm to fill the gap

Other spectrum view do not use the field.

[Brumi-2021]

Hello , good morning , wait , do not go so fast . In my opinion , just under REC in WFM mode (Audio App) that freq that starts at 0 Hz is moving the below red pointer of the baseband demodulated spectrum . It is quite useful to confirm that you have 19Khz stereo subcarrier tone or other audio baseband spectrum area … it is a pity that only reach 0 …48khz . It would be even more interesting if reached 57khz RDS BPSK subcarrier , but currrently still not. Are we talking about that freq. audio baseband cursor ? If yes , I think is really useful .