simplifying symbol table scopes

Linker cannot register extern import symbols initially because
the import object files are not available until all other object files are
resolved. To work around this, we create a stub weak symbol and assign
it to each import symbol. This allows linker to proceed. Once the import
object files are generated, linker replaces the stub symbol with
the actual ones from the import objects.

src/linker/lnk_symbol_table.h

@@ -8,7 +8,6 @@
 typedef enum
 {
   LNK_SymbolScope_Defined,
-  LNK_SymbolScope_Import,
   LNK_SymbolScope_Lib,
   LNK_SymbolScope_Count
 } LNK_SymbolScope;
@@ -30,18 +29,12 @@ typedef struct LNK_SymbolUndefined
   struct LNK_Obj *obj;
 } LNK_SymbolUndefined;
 
-typedef struct LNK_SymbolImport
-{
-  String8 import_header;
-} LNK_SymbolImport;
-
 typedef struct LNK_Symbol
 {
   String8 name;
   union {
     LNK_SymbolDefined   defined;
     LNK_SymbolLib       lib;
-    LNK_SymbolImport    imp;
     LNK_SymbolUndefined undef;
   } u;
 } LNK_Symbol;
@@ -120,7 +113,6 @@ typedef struct
 internal LNK_Symbol * lnk_make_defined_symbol(Arena *arena, String8 name, struct LNK_Obj *obj, U32 symbol_idx);
 internal LNK_Symbol * lnk_make_lib_symbol(Arena *arena, String8 name, struct LNK_Lib *lib, U64 member_offset);
 internal LNK_Symbol * lnk_make_undefined_symbol(Arena *arena, String8 name, struct LNK_Obj *obj);
-internal LNK_Symbol * lnk_make_import_symbol(Arena *arena, String8 name, String8 import_header);
 
 // --- Symbol Containers ------------------------------------------------------