quic: fix lint error in node_quic_crypto

Currently, the following lint error is generated:
Running C++ linter...
File "src/quic/node_quic_crypto.cc" does not use "String"
make[1]: *** [Makefile:1336: tools/.cpplintstamp] Error 1

This commit removes the unused using declaration.

PR-URL: https://github.com/nodejs/node/pull/34019


Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: James M Snell <jasnell@gmail.com>