riscv: defconfig: Remove CONFIG_SND_SOC_STARFIVE=m

The SND_SOC_STARFIVE Kconfig symbol was removed, but it is still enabled
in the RISC-V defconfig.  Just remove it, as it is no longer needed.

Fixes: acc317e5 ("ASoC: starfive: Standardize ASoC menu")
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Conor Dooley <conor.dooley@microchip.com>