Langkit_Support: remove the Functional_Lists package

It was introduced for Adalog's symbolic solver, which no longer uses it,
so remove it from Langkit_Support.

TN: SB20-024

Author: pmderodat

support/langkit_support-functional_lists.adb

@@ -40,7 +40,6 @@ library project Langkit_Support is
       "Langkit_Support.Generic_API.Analysis",
       "Langkit_Support.Generic_API.Introspection",
       "Langkit_Support.File_Readers",
-      "Langkit_Support.Functional_Lists",
       "Langkit_Support.Hashes",
       "Langkit_Support.Images",
       "Langkit_Support.Internal",