closes bpo-42030: Remove legacy AIX dynload support (GH-22717)

Since c19c5a6, AIX builds have defaulted to using dynload_shlib over
dynload_aix when dlopen is available. This function has been available
since AIX 4.3, which went out of support in 2003, the same year the
previously referenced commit was made. It has been nearly 20 years
since a version of AIX has been supported which has not used
dynload_shlib so there's no reason to keep this legacy code around.