Resolve "Remove duplicated code in core-encoder: ACELP excitation coding"

• Related issues: #2304 (closed)
• Requested reviewers: @vaillancour

Reason why this change is needed

• The core-encoder duplicates several functions, namely in excitation encoding in the ACELP core.

Description of the change

• Harmonize excitation encoding in the ACELP core, consequently the duplicated code is removed.
• Removes cca 3,400 lines of the source code.

Affected operating points

• All is BE.

Closes