adbd: delete unused variable.

Bug: https://issuetracker.google.com/150827486
Test: m adb com.android.adbd.apex
Change-Id: I90e10c74d695659c702dea4173cb33ac5ca1a102

Author: jmgao

adb/daemon/file_sync_service.cpp

@@ -591,7 +591,6 @@ static bool do_send_v2(int s, const std::string& path, std::vector<char>& buffer
 static bool recv_uncompressed(borrowed_fd s, unique_fd fd, std::vector<char>& buffer) {
     syncmsg msg;
     msg.data.id = ID_DATA;
-    std::optional<BrotliEncoder<SYNC_DATA_MAX>> encoder;
     while (true) {
         int r = adb_read(fd.get(), &buffer[0], buffer.size() - sizeof(msg.data));
         if (r <= 0) {