net: wireless: bcmdhd: remove obsoleted wl_cfgvendor_priv_string_handler

completely replaced to wl_cfgvendor_get_feature_set_matrix
and it's not used anymore.

Signed-off-by: Insun Song <insun.song@broadcom.com>
Signed-off-by: Siqi Lin <siqilin@google.com>
Bug: 71359108
Change-Id: Iaf6f99842e2925202a800f96330a182f3481f114