Revert commit v3.13.0-rc1~441^2 (install: Teach CODE,SCRIPT modes to
evaluate generator expressions, 2018-05-29).  Unfortunately it has
been found to break existing code in a real project, e.g.

    install(CODE [[
        message("$<FOOBAR>")
    ]])

Address this regression by reverting support for the 3.13 release
series.  Support can be restored later with a policy for compatibility.

Issue: #15785
Fixes: #18435