Clean out left-overs in arguments.hpp

The refactoring done in JDK-8189171 moved some functions out from Arguments to separate classes, but the function declarations were left. These should just be removed.

diff --git a/src/hotspot/share/runtime/arguments.hpp b/src/hotspot/share/runtime/arguments.hpp
--- a/src/hotspot/share/runtime/arguments.hpp
+++ b/src/hotspot/share/runtime/arguments.hpp
@@ -373,13 +373,7 @@
 
   // Tiered
   static void set_tiered_flags();
- // CMS/ParNew garbage collectors
- static void set_parnew_gc_flags();
- static void set_cms_and_parnew_gc_flags();
- // UseParallel[Old]GC
- static void set_parallel_gc_flags();
- // Garbage-First (UseG1GC)
- static void set_g1_gc_flags();
+
   // GC ergonomics
   static void set_conservative_max_heap_alignment();
   static void set_use_compressed_oops();