zink: remove descriptor-mode selection infrastructure

We only support lazy descriptors these days, so having the
infrastructure around to support automatic selection of that one mode is
kinda silly.

And it's not like setting an environment variable that is never read is
going to cause any issues, so we don't even need this to avoid breaking
existing setups.

Let's just rip it out. We can reintroduce it again on the off-chance
that someone has a new clever descriptor mode they want to experiment
with.

Reviewed-by: Hoe Hao Cheng <haochengho12907@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/19670>

docs/drivers/zink.rst

@@ -247,19 +247,6 @@ are required to be supported
 
   * :ext:`VK_KHR_draw_indirect_count`
 
-Performance
------------
-
-If you notice poor performance and high CPU usage while running an application,
-changing the descriptor manager may improve performance:
-
-.. envvar:: ZINK_DESCRIPTORS <mode> ("auto")
-
-``auto``
-   Automatically detect best mode. This is the default.
-``lazy``
-   Attempt to use the least amount of CPU by binding descriptors opportunistically.
-
 Debugging
 ---------