Remove duplicated code: acelp_core_enc_fx()

• Related issues: #2400 (closed)
• Requested reviewers:

Reason why this change is needed

• The core-coder duplicates the main ACELP encoder function acelp_core_enc_fx()

Description of the change

• Remove duplicated main ACELP encoder function
• Removes cca 500 lines of source code

Affected operating points

• All is BE.

Closes #2400 (closed)