[lldb] Cleanup Python API reference files after building the docs

The sphinx-automodapi extension requires that the generated RST files
live next to the index file. This means that we generate them in the
source directory rather than the build directory. This patch ensures
these files are removed again when sphinx finishes its build.

The proper solution to this problem would be to move everything in the
doc folder from the source directory to the build directory before
generating the docs.

I believe that old RST files being kept around is the reason that the
Python API references on the website isn't getting updated. This patch
is meant as a speculative fix and a way to confirm that.