Remove obsolete dead code.

Summary:
Dead because:

$ git grep GETHEADERS_VERS
src/version.h:static const int GETHEADERS_VERSION = 31800;
src/version.h:static const int MIN_PEER_PROTO_VERSION = GETHEADERS_VERSION;

and also because at this line we are guaranteed nVersion >= MIN_PEER_PROTO_VERSION

Test Plan: By observation as described above.

Reviewers: deadalnix, #bitcoin_abc, dagurval, freetrader

Reviewed By: #bitcoin_abc, dagurval, freetrader

Differential Revision: https://reviews.bitcoinabc.org/D333