Remove class path caching and use always use boot's path

Boot is conceptually different than lein, and the way it organizes its
running REPL classpath dynamically means:

- It is not possible to get the exact same classpath as a running REPL
  from a seperate boot invocation. (In particular, because boot can
  dynamically add dependencies to a running process.)
- It is inexpensive to ask boot for what its current classpath is.

On account of this, it seems prudent to simplify vim-boot and remove
caching of classpaths altogether.