[DWARF] Remove extractFast (NFC)

The last use was removed on Dec 13, 2016 in commit
c8c1032.  This patch repurposes the
function comment for the other variant of extractFast.

Author: kazutakahirata

llvm/include/llvm/DebugInfo/DWARF/DWARFDebugInfoEntry.h

@@ -36,8 +36,6 @@ class DWARFDebugInfoEntry {
   /// Extracts a debug info entry, which is a child of a given unit,
   /// starting at a given offset. If DIE can't be extracted, returns false and
   /// doesn't change OffsetPtr.
-  bool extractFast(const DWARFUnit &U, uint64_t *OffsetPtr);
-
   /// High performance extraction should use this call.
   bool extractFast(const DWARFUnit &U, uint64_t *OffsetPtr,
                    const DWARFDataExtractor &DebugInfoData, uint64_t UEndOffset,

llvm/lib/DebugInfo/DWARF/DWARFDebugInfoEntry.cpp

@@ -19,13 +19,6 @@
 using namespace llvm;
 using namespace dwarf;
 
-bool DWARFDebugInfoEntry::extractFast(const DWARFUnit &U,
-                                             uint64_t *OffsetPtr) {
-  DWARFDataExtractor DebugInfoData = U.getDebugInfoExtractor();
-  const uint64_t UEndOffset = U.getNextUnitOffset();
-  return extractFast(U, OffsetPtr, DebugInfoData, UEndOffset, 0);
-}
-
 bool DWARFDebugInfoEntry::extractFast(const DWARFUnit &U, uint64_t *OffsetPtr,
                                       const DWARFDataExtractor &DebugInfoData,
                                       uint64_t UEndOffset, uint32_t D) {