install: Revert CODE,SCRIPT support for generator expressions

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

Source/cmInstallScriptGenerator.h

@@ -6,13 +6,10 @@
 #include "cmConfigure.h" // IWYU pragma: keep
 
 #include "cmInstallGenerator.h"
-#include "cmScriptGenerator.h"
 
 #include <iosfwd>
 #include <string>
 
-class cmLocalGenerator;
-
 /** \class cmInstallScriptGenerator
  * \brief Generate target installation rules.
  */
@@ -23,18 +20,10 @@ public:
                            const char* component, bool exclude_from_all);
   ~cmInstallScriptGenerator() override;
 
-  void Compute(cmLocalGenerator* lg) override;
-
 protected:
-  void GenerateScriptActions(std::ostream& os, Indent indent) override;
-  void GenerateScriptForConfig(std::ostream& os, const std::string& config,
-                               Indent indent) override;
-  void AddScriptInstallRule(std::ostream& os, Indent indent,
-                            std::string const& script);
-
+  void GenerateScript(std::ostream& os) override;
   std::string Script;
   bool Code;
-  cmLocalGenerator* LocalGenerator;
 };
 
 #endif