Revert "[OpenMP][OpenMPIRBuilder] Implement loop unrolling."

Breaks build with -DBUILD_SHARED_LIBS=ON
```
CMake Error: The inter-target dependency graph contains the following strongly connected component (cycle):
  "LLVMFrontendOpenMP" of type SHARED_LIBRARY
    depends on "LLVMPasses" (weak)
  "LLVMipo" of type SHARED_LIBRARY
    depends on "LLVMFrontendOpenMP" (weak)
  "LLVMCoroutines" of type SHARED_LIBRARY
    depends on "LLVMipo" (weak)
  "LLVMPasses" of type SHARED_LIBRARY
    depends on "LLVMCoroutines" (weak)
    depends on "LLVMipo" (weak)
At least one of these targets is not a STATIC_LIBRARY.  Cyclic dependencies are allowed only among static libraries.
CMake Generate step failed.  Build files cannot be regenerated correctly.
```

This reverts commit 707ce34b.