Ignore unsed import in dap/__init__.py

flake8 warns about dap/__init__.py because it has a number of unused
imports.  Most of these are intentional: the import is done to ensure
that the a DAP request is registered with the server object.

This patch applies a "noqa" comment to these imports, and also removes
one import that is truly unnecessary.

Approved-By: Simon Marchi <simon.marchi@efficios.com>