HFP: Remove redundant AG data callout code

* Methods in bta_ag_co.cc are almost empty and should be removed
* Only bta_ag_co_init() calls BTM_WriteVoiceSettings(). This can be
  called directly from BTA. The callout wrapper is unnecessary

Bug: 70538124
Test: runtest bluetooth, native unit tests, no functional change
Change-Id: I20ab1c033d18f3405e7efb8446b44153b24f3cb0