chore: update go.mod and go.sum files (#3721)

This change was generated by running "go mod tidy". No new dependencies are
added to the Kythe project itself by this (and thus no changes to external.bzl).

The net effect is to update go.mod to record some indirect dependences the Go
tool needs, to remove some module deps that are no longer used, and update some
missing hashes in the go.sum file.
2 files changed