Remove duplicated code in core-encoder: non-linearity and LP-CNG

The core-coder duplicates several functions around non_linearity_fx() used for TD BWE. This issue is to track the harmonization of duplications in functions such as

• non_linearity_fx() vs. non_linearity_ivas_fx()
• non_linearity_scaled_copy() vs. non_linearity_scaled_copy_ivas()
• cng_params_upd_fx() vs. cng_params_upd_ivas_fx()
• calculate_hangover_attenuation_gain_fx() vs. calculate_hangover_attenuation_gain_ivas_fx()
• cng_params_upd_fx() vs. cng_params_upd_ivas_fx()
• CNG_enc_fx() vs. CNG_enc_ivas_fx()
• shb_CNG_encod_fx() vs. shb_CNG_encod_ivas_fx()
• swb_CNG_dec_fx() vs. swb_CNG_dec_ivas_fx()

The harmonization is a pre-step for the harmonization of acelp_core_enc_fx().