[AMDGPU] Remove detection of hip runtime for Spack (#133263)

There is special logic to detect the hip runtime when llvm is installed
with Spack. It works by matching the install prefix of llvm against
`llvm-amdgpu-*` followed by effectively globbing for

```
<llvm dir>/../hip-x.y.z-*/
```

and checking there is exactly one such directory.

I would suggest to remove autodetection for the following reasons:

1. In the Spack ecosystem it's by design that every package lives in
   its own prefix, and can only know where its dependencies are
   installed, it has no clue what its dependents are and where they are
installed. This heuristic detection breaks that invariant, since `hip`
   is a dependent of `llvm`, and can be surprising to Spack users.
2. The detection can lead to false positives, since users can be using
   an llvm installed "upstream" with their own build of hip locally, and
   they may not realize that clang is picking up upstream hip instead of
   their local copy.
3. It only works if the directory name is `llvm-amdgpu-*` which happens
   to be the name of AMD's fork of `llvm`, so it makes no sense that
   this code lives in the main LLVM repo for which the Spack package
   name is `llvm`. Feels wrong that LLVM knows about Spack package
   names, which can change over time.
4. Users can change the install directory structure, meaning that this
   detection is not robust under config changes in Spack.

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