Remove duplicated code: core-coder DTX

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

• dtx_fx() vs. dtx_ivas_fx()
• td_cng_enc_init_fx() vs. td_cng_enc_init_ivas_fx()
• dtx_hangover_addition_fx() vs dtx_hangover_addition_fx()
• StableHighPitchDetect_fx() vs. StableHighPitchDetect_fx()

In addition, mallocs and frees are removed from pre_proc_front_ivas_fx().