Consider removing the build script

[philpax]

As part of #96, I ran into two issues:

• my initial clone of the repository resulted in src/themes.rs being emptied out as I didn't have whiskers installed
• the CI builds for my application depending on my fork failed, as the environment didn't have whiskers installed

I've fixed the latter by creating another branch that deletes the build script and depending on that, but I'd like to suggest removing the build script in mainline too to avoid issues like this. The behaviour of this particular automation will be dependent on the state of the developer's system / CI at the time of execution; IMO, any updates to src/themes.rs should be a deliberate action.

As an alternate proposal, the issues I'm encountering come from the dependency upon whiskers in the system environment. Another solution would be to use whiskers as a build dependency (it looks it has some support for library use?) and always call it, so that there's no distinction in behaviour between the-crate-as-in-the-repo and the-crate-as-on-crates-io. This would behave deterministically, but it'd also be an unnecessary cost for something that changes once in a blue moon.