Nit in net_processing.cpp

Summary: There is no point taking the txid, putting it in a CInv, and then reconstructing the TxId using the CInv.

Test Plan:

make check
./test/functional/test_runner.py

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D4575