[fix] update buildenv

CONTACT_URL is unset in Makefile, but searx/brand.py and
utils/brand.env are not updated.

This commit fixes this issue.

searx/brand.py

@@ -4,4 +4,4 @@ ISSUE_URL = 'https://github.com/searx/searx/issues'
 SEARX_URL = 'https://searx.me'
 DOCS_URL = 'https://searx.github.io/searx'
 PUBLIC_INSTANCES = 'https://searx.space'
-CONTACT_URL = 'mailto:contact@example.com'
+CONTACT_URL = ''

utils/brand.env

@@ -4,4 +4,4 @@ export ISSUE_URL='https://github.com/searx/searx/issues'
 export SEARX_URL='https://searx.me'
 export DOCS_URL='https://searx.github.io/searx'
 export PUBLIC_INSTANCES='https://searx.space'
-export CONTACT_URL='mailto:contact@example.com'
+export CONTACT_URL=''