[SYCL] Update build instructions after latest sync with llorg.

LLVM_EXTERNAL_PROJECTS variable is not required anymore.

Signed-off-by: Alexey Bader <alexey.bader@intel.com>

sycl/doc/GetStartedWithSYCLCompiler.md

@@ -44,7 +44,7 @@ runtime use modified CMake command below:
 mkdir $SYCL_HOME/build
 cd $SYCL_HOME/build
 cmake -DCMAKE_BUILD_TYPE=Release -DOpenCL_INCLUDE_DIR=$OPENCL_HEADERS \
--DLLVM_ENABLE_PROJECTS="clang" -DLLVM_EXTERNAL_PROJECTS="sycl;llvm-spirv" \
+-DLLVM_ENABLE_PROJECTS="clang" \
 -DLLVM_EXTERNAL_SYCL_SOURCE_DIR=$SYCL_HOME/sycl \
 -DLLVM_EXTERNAL_LLVM_SPIRV_SOURCE_DIR=$SYCL_HOME/llvm-spirv \
 -DLLVM_TOOL_SYCL_BUILD=ON -DLLVM_TOOL_LLVM_SPIRV_BUILD=ON $SYCL_HOME/llvm