Meta: Don't pass LLVM_LLVM_BUILD_LLVM_DYLIB to cmake

`LLVM_LLVM_BUILD_LLVM_DYLIB` does not exist, so passing this does
nothing but make CMake warn.

However, since we pass `LLVM_LINK_LLVM_DYLIB`, `LLVM_BUILD_LLVM_DYLIB`
(the correct spelling) defaults to true anyways.  So let's pass fewer
flags.

No behavior change, but fixes a CMake warning.

Toolchain/BuildClang.sh

@@ -259,7 +259,6 @@ pushd "$DIR/Build/clang/$ARCH"
  -DLLVM_ENABLE_PROJECTS="clang;lld" \
  -DLLVM_INCLUDE_BENCHMARKS=OFF \
  -DLLVM_INCLUDE_TESTS=OFF \
- -DLLVM_LLVM_BUILD_LLVM_DYLIB=ON \
  -DLLVM_LINK_LLVM_DYLIB=ON \
  -DLLVM_INSTALL_UTILS=OFF \
  ${dev:+"-DLLVM_CCACHE_BUILD=ON"} || exit 1