[mod] remove option ui.static_use_hash (cache busting) (#5004)

Cache busting has caused serious problems for users in the past, here are two
examples:

- https://github.com/searxng/searxng/issues/4419
- https://github.com/searxng/searxng/issues/4481

And it makes development and deployment significantly more complex because it
binds the client side to the server side:

- https://github.com/searxng/searxng/pull/4466

In the light of a decoupled development of the WEB clients from the server side:

- https://github.com/searxng/searxng/pull/4988

is it appropriate to abandon this feature. In fact,  it has been ineffective
since #4436 anyway.

However, the benefit has always been questionable, since at best only a few kB
of data are saved (at least in the context of an image_proxy, the effect is below
the detection limit). Ultimately, the client is responsible for caching.

Related: https://github.com/searxng/searxng/issues?q=label%3A%22clear%20browser%20cache%22

Closes: https://github.com/searxng/searxng/pull/4466
Closes: https://github.com/searxng/searxng/issues/1326
Closes: https://github.com/searxng/searxng/issues/964

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

docs/admin/settings/settings_ui.rst

@@ -10,7 +10,6 @@
 .. code:: yaml
 
    ui:
-     static_use_hash: false
      default_locale: ""
      query_in_title: false
      infinite_scroll: false
@@ -23,11 +22,6 @@
      hotkeys: default
      url_formatting: pretty
 
-.. _static_use_hash:
-
-``static_use_hash`` : ``$SEARXNG_STATIC_USE_HASH``
-  Enables `cache busting`_ of static files.
-
 ``default_locale`` :
   SearXNG interface language.  If blank, the locale is detected by using the
   browser language.  If it doesn't work, or you are deploying a language