Removed obsolete file.

I wrote:
> Moved from last cc65 release (2.13.3) to recent cc65 snapshot (2.13.9).
> [...]
> Atari:
> - The builtin linker config allows to override the start addr so there no more need for a custom linker config.
> [...]
However I didn't actually remove the custom linker config not needed anymore.
Merge pull request #260 from oliverschmidt/master

Removed obsolete file.
Added platform 'Atari XL'.

Don't be afraid, I'm not trying to have more retro platforms than "real" platforms ;-)
The platform 'atarixl' will replace the platform 'atari'. However I need both for some
transition period.
Merge pull request #261 from oliverschmidt/master

Added platform 'Atari XL'.
Another small addition for platform 'Atari XL'.
Merge pull request #263 from oliverschmidt/master

Another small addition for 'Atari XL'.