Remove globals.h use from net.cpp

Summary: The only place where it is used is to compute a buffer size, and it doesn't really need to be used there. ONE8MEGABYTE is good enough.

Test Plan:

make check
../qa/pull-tester/rpc-tests.py maxuploadtarget.py

Reviewers: freetrader, sickpig, kyuupichan, awemany, #bitcoin_abc

Reviewed By: freetrader, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D204