Remove unused "highlight-bash" script

Unused since bf208de2. The original commit message of "highlight-bash"
said:

> The "highlight-bash" perl script is quite nasty. If anyone has to
> maintain this, I give you permission to get rid of the "highlighting"
> altogether (actually, it's linking to reference documentation for shell
> commands and bash builtins -- I thought it would be nice to try some
> unobtrusive documentation).

It was a bit of an experiment and probably not actually useful to
anybody, and too much effort to maintain.
Remove obsolete files from "make clean" recipe