Main: Remove leftover test files in lib_util (test_mdct.c, test_fft.c)

Basic info

• Fixed point: 3782eb08

Bug description

The main branch has several test files which seem to be leftovers. These should be removed:

• https://forge.3gpp.org/rep/sa4/audio/ivas-basop/-/blob/main/lib_util/test_mdct.c
• https://forge.3gpp.org/rep/sa4/audio/ivas-basop/-/blob/main/lib_util/test_fft.c
• https://forge.3gpp.org/rep/sa4/audio/ivas-basop/-/blob/main/lib_util/test_fft.h