xdist.plugin: Remove unnecessary type for --dist

1e18f25 corrected an issue with the `type` specification for the `--dist`
argument, setting it to `str`. However, this is the default so there's little
value in specifying it.

xdist/plugin.py

@@ -32,7 +32,7 @@ def pytest_addoption(parser):
     group._addoption(
         '--dist', metavar="distmode",
         action="store", choices=['load', 'each', 'no'],
-        type=str, dest="dist", default="no",
+        dest="dist", default="no",
         help=("set mode for distributing tests to exec environments.\n\n"
               "each: send each test to each available environment.\n\n"
               "load: send each test to available environment.\n\n"