Upgraded termite to v8, and its dependency vte to v0.38.0

I've removed unnecessary dependencies. They were mistakenly added in https://github.com/k0ral/nixpkgs/commit/2a56f3d3f3a90cfae59a0ce89c0d1cc7d2e610f0 . Only `vte-0.38.0` requires `vala` and `libxml2`