HFP: Remove AG RFCOMM data pass through mode

* In BTA_AG_PASS_THROUGH mode defined in bta_ag_api.h, RFCOMM data is
  passed directly to the upper layer. In this case, AT commands are
  parsed in the user application instead of being parsed within the fluoride
  stack.
* However, this mode is never used on the stack as btif_hf.cc only call
  BTA_AgEnable() with BTA_AG_PARSE parameter value. Currently, AT
  commands are only parsed within the fluoride stack
* This change removes this feature since the pass through mode is
  never used, tested, nor maintained

Bug: 70538124
Test: runtest bluetooth, native unit tests, no functional change
Change-Id: I921d5ee950e4abadea8433c5b2d5882a99923cb0