Remove various duplicated code in core-coder

This issue is to track the harmonization of duplications in the following core-coder functions:

• gain_dec_tc_fx() vs. gain_dec_tc_ivas_fx()
• encode_lpc_avq_fx() vs. encode_lpc_avq_ivas_fx()
• decode_lpc_avq_fx() vs. decode_lpc_avq_ivas_fx()
• lsf_bctcvq_encprm_fx() vs. lsf_bctcvq_encprm_ivas_fx()
• lsf_msvq_ma_encprm_fx() vs. lsf_msvq_ma_encprm_ivas_fx()
• calc_st_filt_local_fx() vs. calc_st_filt_ivas_fx()
• concealment_init_fx() vs. concealment_init_ivas_fx()
• Filt_mu_fx() vs. Filt_mu_ivas_fx()