Remove unused dependencies (#85)

https://github.com/est31/cargo-udeps reports "thiserror" as unused
by all targets:

```shell
$ cargo +nightly udeps --all-targets
unused dependencies:
`helm-templexer v2.0.0 (/home/hmaus/Development/helm-templexer)`
└─── dependencies
     └─── "thiserror"
```

Author: hendrikmaus

Cargo.lock

@@ -21,7 +21,6 @@ structopt-flags = "0.3"
 log = "0.4"
 env_logger = "0.8"
 anyhow = "1"
-thiserror = "1"
 serde = "1"
 serde_yaml = "0.8"
 subprocess = "0.2"