travis: Remove Travis unwanted code.

Since in travis we don't have root access so we used to add different
srv path. As now we shifted our production suites to Circle CI
we don't need that code so removed it.

Also we used a hacky code in commit-lint-message for travis which is
now of no use.

scripts/lib/clean_venv_cache.py

@@ -15,8 +15,6 @@ from scripts.lib.zulip_tools import \
 
 ENV = get_environment()
 VENV_CACHE_DIR = '/srv/zulip-venv-cache'
-if ENV == "travis":
-    VENV_CACHE_DIR = os.path.join(os.environ["HOME"], "zulip-venv-cache")
 
 def get_caches_in_use(threshold_days: int) -> Set[str]:
     setups_to_check = {ZULIP_PATH}