docs: drop custom envvar directive

The custom envvar directive is strange. It overloads the global
namespace, where there's already a well-known envvar directive, but the
envvar role won't resolve to envvars defined using the custom directive.

It's not really critical to declare the type and default value here, and
many descriptions already include this. Let's just use the plain one,
and let the description take care of the details where needed.

Reviewed-by: Yonggang Luo <luoyonggang@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/19593>

docs/_exts/formatting.py

@@ -9,18 +9,6 @@ import sphinx.addnodes
 from sphinx.util.nodes import split_explicit_title
 from docutils import nodes, utils
 
-def parse_envvar(env, sig, signode):
-    envvar, t, default = sig.split(" ", 2)
-    envvar = envvar.strip().upper()
-    t = "Type: %s" % t.strip(" <>").lower()
-    default = "Default: %s" % default.strip(" ()")
-    signode += sphinx.addnodes.desc_name(envvar, envvar)
-    signode += docutils.nodes.Text(' ')
-    signode += sphinx.addnodes.desc_type(t, t)
-    signode += docutils.nodes.Text(', ')
-    signode += sphinx.addnodes.desc_annotation(default, default)
-    return envvar
-
 def parse_opcode(env, sig, signode):
     opcode, desc = sig.split("-", 1)
     opcode = opcode.strip().upper()
@@ -46,8 +34,6 @@ def ext_role(name, rawtext, text, lineno, inliner, options={}, content=[]):
     return [pnode], []
 
 def setup(app):
-    app.add_object_type("envvar", "envvar", "%s (environment variable)",
-        parse_envvar)
     app.add_object_type("opcode", "opcode", "%s (TGSI opcode)",
         parse_opcode)
     app.add_role('ext', ext_role)