RFR 8231990: Remove unnecessary else-if branch in Types.union

[Ron Shapiro]

Please review the following tiny patch which was already discussed/review
in another thread[0]

webrev: http://cr.openjdk.java.net/~ronsh/8231990/webrev.00/
bug: https://bugs.openjdk.java.net/browse/JDK-8231990

The patch was prepared by Brad Corso (cc'ed) - whoever ends up submitting,
can you make sure that the authorship is marked correctly? Thanks!

[0]:
https://mail.openjdk.java.net/pipermail/compiler-dev/2019-September/013679.html
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