header-checker: Remove OpenMP workaround

This commit removes OpenMP workaround for old clang.  clang-r339409b
includes the latest `omp.h`.

Bug: 117764794
Bug: 111579848  (Port header-abi-dumper to clang-r339409b)
Bug: 77161122   (The bug that introduced the workaround)
Test: development/vndk/tools/header-checker/utils/create_reference_dumps.py
Test: libneuralnetworks.so.lsdump is created successfully.
Change-Id: Id65229551f86545e77bd8932b0ed4bf4e6783694