Remove ZeroPadExtentWriter.

We don't generate replace operations with trailing zero removed, so
we can remove ZeroPadExtentWriter and ExtentWriter::End().

Bug: 78792859
Test: update_engine_unittests
Test: brillo_update_payload verify
Change-Id: I3c36d80a9c4475fda0b32c86c6503ab8b179b88f
diff --git a/payload_consumer/fake_extent_writer.h b/payload_consumer/fake_extent_writer.h
index 4418a9e..7b2b7ac 100644
--- a/payload_consumer/fake_extent_writer.h
+++ b/payload_consumer/fake_extent_writer.h
@@ -40,26 +40,20 @@
     return true;
   };
   bool Write(const void* bytes, size_t count) override {
-    if (!init_called_ || end_called_)
+    if (!init_called_)
       return false;
     written_data_.insert(written_data_.end(),
                          reinterpret_cast<const uint8_t*>(bytes),
                          reinterpret_cast<const uint8_t*>(bytes) + count);
     return true;
   }
-  bool EndImpl() override {
-    end_called_ = true;
-    return true;
-  }
 
   // Fake methods.
   bool InitCalled() { return init_called_; }
-  bool EndCalled() { return end_called_; }
   brillo::Blob WrittenData() { return written_data_; }
 
  private:
   bool init_called_{false};
-  bool end_called_{false};
   brillo::Blob written_data_;
 
   DISALLOW_COPY_AND_ASSIGN(FakeExtentWriter);