Revert "Check for invalid projects passed in LLVM_ENABLE_PROJECTS"

```
CMake Error at CMakeLists.txt:76 (MESSAGE):
  all isn't a know project:
  clang;clang-tools-extra;compiler-rt;cross-project-tests;libc;libclc;libcxx;libcxxabi;libunwind;lld;lldb;mlir;openmp;parallel-libs;polly;pstl;flang;llvm
```

This reverts commit 4121ac1e.