Drop adding Android Verified Boot support

AVB is a fine feature, but it isn't actually needed for the purposes of
enabling UEFI secure variables via StMM.
Signed-off-by: Grant Likely <grant.likely@arm.com>

stmm.config

 CONFIG_TEE=y
 CONFIG_OPTEE=y
-CONFIG_OPTEE_TA_AVB=y
 CONFIG_EFI_MM_COMM_TEE=y
 CONFIG_SUPPORT_EMMC_RPMB=y