msm8960-common: remove unused (and incorrect) flag

* TARGET_ARCH_VARIANT_CPU is only used for armv6
* TARGET_USE_KRAIT_BIONIC_OPTIMIZATION is deprecated

See here: https://github.com/CyanogenMod/android_device_htc_msm8960-common/commit/52784366241b7f69077e60eaa82b6b0bac3caa4f

Change-Id: I1958199076b4e7b5be37f77260c819d814de9c69