Remove shebang line from trac*/**/.py source files

[Jun Omae]

Considering #13765, the following files have shebang line, however I think the shebang is not needed because we and a user can run it with python -m <MODULE> and python -m unittest <MODULE>.test_suite.

$ git grep -n '^#!' -- 'trac*' | grep ':1:'
trac/admin/console.py:1:#!/usr/bin/env python3
trac/admin/tests/functional.py:1:#!/usr/bin/env python3
trac/db/tests/functional.py:1:#!/usr/bin/env python3
trac/mimeview/tests/functional.py:1:#!/usr/bin/env python3
trac/prefs/tests/functional.py:1:#!/usr/bin/env python3
trac/search/tests/functional.py:1:#!/usr/bin/env python3
trac/test.py:1:#!/usr/bin/env python3
trac/tests/functional/__init__.py:1:#!/usr/bin/env python3
trac/tests/functional/testcases.py:1:#!/usr/bin/env python3
trac/ticket/tests/functional/admin.py:1:#!/usr/bin/env python3
trac/ticket/tests/functional/default_workflow.py:1:#!/usr/bin/env python3
trac/ticket/tests/functional/main.py:1:#!/usr/bin/env python3
trac/timeline/tests/functional.py:1:#!/usr/bin/env python3
trac/versioncontrol/tests/functional.py:1:#!/usr/bin/env python3
trac/web/cgi_frontend.py:1:#!/usr/bin/env python3
trac/web/fcgi_frontend.py:1:#!/usr/bin/env python3
trac/web/standalone.py:1:#!/usr/bin/env python3
trac/wiki/tests/functional.py:1:#!/usr/bin/env python3