[HIP] remove overloaded abs in header

This function seems to be introduced by accident by
https://github.com/llvm/llvm-project/commit/aa2b593f1495a972a4a592952760ec9d5f7c01f1

Such overloaded abs function did not exist before
the refactoring, and does not exist in
https://github.com/llvm/llvm-project/blob/main/clang/lib/Headers/__clang_cuda_cmath.h

Conceptually it also does not make sense, since it adds something like

double abs(int x) {
  return ::abs((double)x);
}

It caused regressions in CuPy.

Reviewed by: Aaron Enye Shi, Artem Belevich

Differential Revision: https://reviews.llvm.org/D99738