tests: Remove slow tests detection.

According to @showell:

> All the slow decorators can die. That was a failed experiment of
> mine from 2014 days.  I have meaning to kill them for a couple years
> now.  I wrote this with the best of intentions, but I believe it's
> now just cruft.  We never made a "fast" mode, for one. And we kept
> writing more and more slow tests, haha.

Signed-off-by: Anders Kaseorg <anders@zulip.com>

tools/test-backend

@@ -241,10 +241,6 @@ def main() -> None:
                         action="store_true",
                         default=False,
                         help="Show detailed output")
-    parser.add_argument('--report-slow-tests', dest='report_slow_tests',
-                        action="store_true",
-                        default=False,
-                        help="Show which tests are slowest.")
     parser.add_argument('--reverse', dest='reverse',
                         action="store_true",
                         default=False,
@@ -458,13 +454,6 @@ def main() -> None:
             print("1.) `vagrant ssh -- -L 8080:127.0.0.1:8080`")
             print(f"2.) `snakeviz -s {shlex.quote(stats_file.name)}`")
 
-    if options.report_slow_tests:
-        from zerver.lib.test_runner import report_slow_tests
-
-        # We do this even with failures, since slowness can be
-        # an important clue as to why tests fail.
-        report_slow_tests()
-
     # Ideally, we'd check for any leaked test databases here;
     # but that needs some hackery with database names.
     #