Revert "Use shorter tracebacks in the JSON report"

This reverts commit f356ffd.

It doesn't actually save that much space, so let's just use the default since
it has more information and it keeps the code simpler.

Author: asmeurer

conftest.py

@@ -126,8 +126,3 @@ def pytest_collection_modifyitems(config, items):
             ci_mark = next((m for m in markers if m.name == "ci"), None)
             if ci_mark is None:
                 item.add_marker(mark.skip(reason="disabled via --ci"))
-
-    # Avoid long tracebacks for the JSON report, which make the file too
-    # large.
-    if config.getoption('--json-report'):
-        config.option.tbstyle = 'native'