Eliminating vm->main_index introduced in ea2ec4c3ed7d.

The vm->main_index was only needed for the disassembler in the
accumulative mode.  It was used to prevent from disassembling the code
from the previous iterations in the CLI.

The same result can be achieved without introducing the VM level field.
diff --git a/src/njs_disassembler.c b/src/njs_disassembler.c
index af99caf..dd67fa6 100644
--- a/src/njs_disassembler.c
+++ b/src/njs_disassembler.c
@@ -146,8 +146,16 @@
     njs_vm_code_t  *code;
 
     code = vm->codes->start;
-    code += vm->main_index;
-    n = vm->codes->items - vm->main_index;
+    n = vm->codes->items;
+
+    while (n != 0) {
+        if (code->start == vm->start) {
+            break;
+        }
+
+        code++;
+        n--;
+    }
 
     while (n != 0) {
         njs_printf("%V:%V\n", &code->file, &code->name);
diff --git a/src/njs_vm.c b/src/njs_vm.c
index a4e708e..366a5e5 100644
--- a/src/njs_vm.c
+++ b/src/njs_vm.c
@@ -156,7 +156,6 @@
         return NJS_ERROR;
     }
 
-    vm->main_index = code - (njs_vm_code_t *) vm->codes->start;
     vm->start = generator.code_start;
     vm->global_scope = generator.local_scope;
     vm->scope_size = generator.scope_size;
diff --git a/src/njs_vm.h b/src/njs_vm.h
index c232908..47f0f87 100644
--- a/src/njs_vm.h
+++ b/src/njs_vm.h
@@ -233,9 +233,7 @@
     njs_object_t             string_object;
     njs_object_t             global_object;
 
-    njs_uint_t               main_index;
     njs_arr_t                *codes;  /* of njs_vm_code_t */
-
     njs_arr_t                *functions_name_cache;
 
     njs_trace_t              trace;