CorebootPayloadPkg: Remove improper build flags in DSC file

CorebootPayloadPkg: Remove improper build flags in DSC file

Current CorebootPayloadPkgIa32X64.dsc contains "-flto" flag to
request GCC link time optimization. However, this feature is
only supported by newer GCC compiler, and it will break the
debug build with GCC4.8. To fix it, the extra compiling flags
are removed. It allows the default build flags set by the EDKII
build environment to be used.

With this fix, CorebootPayloadPkg 64bit debug build can pass
using GCC 4.8.

Cc: Prince Agyeman <prince.agyeman@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Maurice Ma <maurice.ma@intel.com>
Reviewed-by: Liming Gao <liming.gao@intel.com>