Remove duplicated code in core switching

The core-coder duplicates several functions in core switching. This issue is to track the removal of duplications in functions such as

• core_switching_pre_enc_ivas_fx() vs core_switching_pre_enc_fx()
• core_switching_pre_dec_ivas_fx() vs. core_switching_pre_dec_fx()
• core_switching_post_enc_ivas_fx() vs. core_switching_post_enc_fx()
• core_switching_post_dec_ivas_fx() vs. core_switching_post_dec_ivas_fx()
• bandwidth_switching_detect_ivas_fx() vs. bandwidth_switching_detect_fx()
• ivas_bw_switching_pre_proc_fx() vs. bw_switching_pre_proc_fx()