Remove unused length from DexFile GetString calls.

Address extra review comments from commit
dfb325e0ddd746cd8f7c2e3723b3a573eb7cc111.

Change-Id: If76e81e7af5870431901de0bf561e0f827435fe3
diff --git a/runtime/dex_file.cc b/runtime/dex_file.cc
index d3bb483..7e09a48 100644
--- a/runtime/dex_file.cc
+++ b/runtime/dex_file.cc
@@ -457,9 +457,8 @@
   int32_t hi = NumStringIds() - 1;
   while (hi >= lo) {
     int32_t mid = (hi + lo) / 2;
-    uint32_t length;
     const DexFile::StringId& str_id = GetStringId(mid);
-    const char* str = GetStringDataAndUtf16Length(str_id, &length);
+    const char* str = GetStringData(str_id);
     int compare = CompareModifiedUtf8ToModifiedUtf8AsUtf16CodePointValues(string, str);
     if (compare > 0) {
       lo = mid + 1;
@@ -477,9 +476,8 @@
   int32_t hi = NumStringIds() - 1;
   while (hi >= lo) {
     int32_t mid = (hi + lo) / 2;
-    uint32_t length;
     const DexFile::StringId& str_id = GetStringId(mid);
-    const char* str = GetStringDataAndUtf16Length(str_id, &length);
+    const char* str = GetStringData(str_id);
     int compare = CompareModifiedUtf8ToUtf16AsCodePointValues(str, string);
     if (compare > 0) {
       lo = mid + 1;