Clean up extra ifdefs in runtime/include/j9lib.h.in

[sharon-wang]

Mentioned in review of #11166.

There are a bunch of duplicated DLL Name defines in j9lib.h.in, as per https://github.com/eclipse/openj9/blob/05e6d8f3fb20bb8278b146764bf88fd8de7f3f2a/runtime/include/j9lib.h.in#L68

We should look into removing the extra defines so the file can be cleaned up.

CC @keithc-ca @dnakamura

[sharon-wang]

Thanks @dnakamura for the additional insight, from a review comment in #11166:

The comment /* We need certain defines... this is a big hack until we can do this another way.*/ is copied over from j9lib.h.ftl. It seems there may be some UMA limitation, where targets don't get picked up as part of:

<#list uma.spec.artifacts as artifact>
<#if artifact.data.dllDescription.present>
#define J9${artifact.data.dllDescription.underscored_data}_DLL_NAME "${artifact.targetNameWithRelease}"
</#if>
</#list>

It should be safe to delete the extra defines below the /* We need certain defines... this is a big hack until we can do this another way.*/ comment in j9lib.h.in.

It's also not necessary to keep extern "C" since there are only #defines in the file.