htop: fetchFromGitHub -> fetchurl

Just call `fetchurl` on the ‘official’ download link.

`fetchFromGitHub` was introduced in 54916ba6, and stayed around when
we moved back to a regular release. Which worked fine, but required
keeping `autoreconfHook` and some `postPatch` hackery which can now
both be dropped.

Thanks to @kragniz for drawing my attention to this.