[mod] reduce enviroment variables in shell scripts to what is needed

Not all settings from the 'brand:' section of the YAML files are needed in the
shell scripts.  This patch reduce the variables in ./utils/brand.env to what is
needed.  The following ('brand:' settings) can be removed from this file:

- ISSUE_URL
- DOCS_URL
- PUBLIC_INSTANCES
- WIKI_URL

Tasks running outside of an *installed instance*, need the following settings
from the YAML configuration:

- GIT_URL            <--> brand.git_url
- GIT_BRANCH         <--> brand.git_branch
- SEARX_URL          <--> server.base_url  (aka PUBLIC_URL)
- SEARX_PORT         <--> server.port
- SEARX_BIND_ADDRESS <--> server.bind_address

Signed-off-by: Markus Heiser <markus.heiser@darmarit.de>

utils/templates/etc/searx/settings.yml

@@ -1,6 +1,6 @@
 # SearXNG settings, before editing this file read:
 #
-#     ${DOCS_URL}/admin/engines/settings.html
+#     https://searxng.github.io/searxng/admin/engines/settings.html
 
 use_default_settings: true