sm8350-common: Drop unnecessary audio HAL flags from BoardConfig

BOARD_USES_ALSA_AUDIO is only used to determine wether the audio HAL
is built from source, but we ship the prebuilt stock HAL.
AUDIO_FEATURE_ENABLED_GEF_SUPPORT is used to modify the source-built
audio HAL, which does not have an effect for us.

Change-Id: I652de533d1c21a0e45dc7406bd0698326c5dbcd7