Remove documentation for defunct standalone SharpClaw code

[ketch]

SharpClaw is no longer supported or available as a standalone code, but only inside of PyClaw. There is still documentation for the standalone code:

https://www.clawpack.org/sharpclaw/doc/index.html

This could be confusing since people might think it refers to pyclaw.sharpclaw. @rjleveque Could you delete that whole directory?