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_node_cache.py

@@ -1,7 +1,6 @@
 #!/usr/bin/env python3
 import argparse
 import os
-import subprocess
 import sys
 
 from typing import Set
@@ -14,14 +13,6 @@ from scripts.lib.zulip_tools import \
 
 ENV = get_environment()
 NODE_MODULES_CACHE_PATH = "/srv/zulip-npm-cache"
-if ENV == "travis":
-    NODE_MODULES_CACHE_PATH = os.path.join(os.environ["HOME"], "zulip-npm-cache")
-    try:
-        subprocess.check_output(["/home/travis/zulip-yarn/bin/yarn", '--version'])
-    except OSError:
-        print('yarn not found. Most probably we are running static-analysis and '
-              'hence yarn is not installed. Exiting without cleaning npm cache.')
-        sys.exit(0)
 
 def get_caches_in_use(threshold_days: int) -> Set[str]:
     setups_to_check = {ZULIP_PATH}