Buck: Remove non working local_jar rule and documentation

Since: [1] local_jar() is broken and cannot be fixed. It was always
a hack that relied on the ability to define two different targets
with the same output artifact name.

* [1] https://github.com/facebook/buck/commit/c92ef212b53fff08a8452649b4d4faadc6b89b11

Change-Id: I5801d71092763bdcee1c07e35aafa5e0d6ca9e39