go.mod: remove obsolete exclude rule (#2077)

The exclude was originally added in 02fec464ad5c015332dcbd4db0cf53b96967e3f2
to prevent traversing client_golang v1.12.1. No current dependency requires
that version, making the exclude obsolete.

To verify;

    go list -m all

which produced no changes.

Also verified that:

    go mod tidy
    go test ./...

produce no changes and all tests pass.

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>