Revert "ibus-packages.mozc: pin to protobuf 3.6"
Revert "ibus-packages.mozc: pin to protobuf 3.6"

protobuf was bumped to 3.7.1, which fixes the build issue as described
in google/mozc#460.

This reverts commit a19fbaf39ba2b9b1434b3dd159c79d0f645f0a6c.

NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs