NFC: Remove spurious forward declarations

There is no such thing as "struct Slab" any more.

We use alignof(RemoteAllocator) below, so we already require the complete type
definition at this point.

src/mem/metaslab.h

@@ -9,8 +9,6 @@
 
 namespace snmalloc
 {
-  class Slab;
-
   // The Metaslab represent the status of a single slab.
   // This can be either a short or a standard slab.
   class alignas(CACHELINE_SIZE) Metaslab
@@ -184,8 +182,6 @@ namespace snmalloc
     }
   };
 
-  struct RemoteAllocator;
-
   /**
    * Entry stored in the pagemap.
    */