Remove USE_CLANG_PLATFORM_BUILD in BoardConfig.mk

Use global default USE_CLANG_PLATFORM_BUILD set in core/envsetup.mk,
or user provided environment variable USE_CLANG_PLATFORM_BUILD.

BUG: 26102335
Change-Id: I7e12219a60f36bb44797bb028b4a5873a67c9210
6 files changed