net: simplify the call to vProcessMsg.splice()

Summary:
At the time when

cpp
pnode->vProcessMsg.splice(pnode->vProcessMsg.end(), pnode->vRecvMsg, pnode->vRecvMsg.begin(), it);

is called, it is certainly pnode->vRecvMsg.end() which makes the
call equivalent to:

cpp
pnode->vProcessMsg.splice(pnode->vProcessMsg.end(), pnode->vRecvMsg, pnode->vRecvMsg.begin(), pnode->vRecvMsg.end());

which is equivalent to:

cpp
pnode->vProcessMsg.splice(pnode->vProcessMsg.end(), pnode->vRecvMsg);

Thus, use the latter. Further, maybe irrelevant, but the latter has
constant complexity while the original code is O(length of vRecvMsg).

This is a backport of core#26888
Depends on D18243

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D18244