install: Revert CODE,SCRIPT support for generator expressions
Revert !2113 (merged). 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 (closed)
Fixes: #18435 (closed)