[clang] clang/HIP: Remove __ockl_fdot2 declaration (PR #201878)

[Matt Arsenault via cfe-commits]

arsenm wrote:

> downstream needs to move away from usage; can you revert for now ?

The usage doesn't need to change, the HIP headers needed to stop conditionally declaring the used functions which https://github.com/ROCm/rocm-systems/pull/2230 did 

https://github.com/llvm/llvm-project/pull/201878