Remove phat_small

[karllark]

No longer used. And has formats of files that are no longer supported by the lastest BEAST code.

[karllark]

When this is done, the example in the BEAST docs and instructions need to be updated to metal_small.

As pointed out by @cmurray-astro : the beast_settings.txt file needs to be updated to include all the great comments in the phat_small version.