Running mypy daemon on internal Dropbox codebase can cause an Assertion
Error:

```
version: 1.16.0+dev.ca609acabdc94ee973a53d62b8dcb7e55c789aec
Daemon crashed!
Traceback (most recent call last):
  File "mypy/dmypy_server.py", line 237, in serve
  File "mypy/dmypy_server.py", line 286, in run_command
  File "mypy/dmypy_server.py", line 364, in cmd_check
  File "mypy/dmypy_server.py", line 428, in check
  File "mypy/dmypy_server.py", line 517, in initialize_fine_grained
  File "mypy/server/update.py", line 265, in update
  File "mypy/server/update.py", line 367, in update_one
  File "mypy/server/update.py", line 432, in update_module
  File "mypy/server/update.py", line 672, in update_module_isolated
  File "mypy/build.py", line 2410, in finish_passes
  File "mypy/build.py", line 2417, in free_state
  File "mypy/checker.py", line 443, in reset
AssertionError
```

Let's convert these asserts in reset() to actual cleanup. I see no
reason not to clean them up.
It also seems safe for this particular crash, since in
File "mypy/build.py", line 2417, in free_state
right after this line there's `self._type_checker = None`. So even if we
wer not to call reset() everything would still be correct.