Remove duplicated code: FD BWE

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

• wb_bwe_enc_ivas_fx() vs wb_bwe_enc_fx()
• swb_bwe_enc_ivas_fx() vs swb_bwe_enc_fx()
• WB_BWE_encoding_ivas_fx() vs WB_BWE_encoding_fx()
• SWB_BWE_encoding_ivas_fx() vs SWB_BWE_encoding_fx()