[avalanche] Remove redundant code to ignore stake contender getdata requests

Summary:
vNotFound appears to be used only for special cases (txs and proofs). Since the default is to ignore, this code is redundant.

There is no change in behavior.

Test Plan:

ninja && ./test/functional/test_runner.py p2p_inv_download.py

Reviewers: O1 Bitcoin ABC, #bitcoin_abc, PiRK

Reviewed By: O1 Bitcoin ABC, #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D17190