Remove use_dsp_codec_when_controller_does_not_support flag

This flag was added in 24Q2 to allow the DSP to be used for LC3 when the BT controller does not support it. This is now the default behavior, so the flag is no longer needed.

Bug: 347316578
Change-Id: I581d217fa07472152cded36f160b964168735328
Flag: exempt, nextfood flag removal change
Test: atest net_test_bta