[SYCL] Remove cg.cpp as it's empty now

Signed-off-by: Sergey Kanaev <sergey.kanaev@intel.com>

Author: Sergey Kanaev

sycl/source/CMakeLists.txt

@@ -101,7 +101,6 @@ set(SYCL_SOURCES
     "detail/builtins_integer.cpp"
     "detail/builtins_math.cpp"
     "detail/builtins_relational.cpp"
-    "detail/cg.cpp"
     "detail/pi.cpp"
     "detail/common.cpp"
     "detail/config.cpp"