zulip/examples: Remove sys.path modifications.

With the new repo, doing a `pip install -e ./zulip` will be a part
of the core workflow. This mitigates the risk of developers
accidentally testing their changes against an installed copy of the
zulip package rather than the copy in their checkout. Therefore,
we can now get rid of the various `sys.path.inserts` in the examples.

zulip/examples/print-messages

@@ -22,8 +22,6 @@
 # THE SOFTWARE.
 
 from __future__ import print_function
-import sys
-import os
 import argparse
 
 from typing import Any, Dict
@@ -36,7 +34,7 @@ Example: print-messages --user=username@example.com --api-key=a0b1c2d3e4f5a6b7c8
 
 You can omit --user and --api-key arguments if you have a properly set up ~/.zuliprc
 """
-sys.path.append(os.path.join(os.path.dirname(__file__), '..'))
+
 import zulip
 
 parser = zulip.add_default_arguments(argparse.ArgumentParser(usage=usage))