radeonsi: remove AMD_DEBUG=sisched option

sisched is not maintained anymore in LLVM.

Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Reviewed-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>
Tested-by: Marge Bot <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/4059>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/4059>

docs/envvars.html

@@ -712,8 +712,6 @@ Mesa EGL supports different sets of environment variables.  See the
   <dt><code>preoptir</code></dt>
   <dd>Print the LLVM IR before initial optimizations</dd>
   <h4>Shader compilation tuning flags:</h4>
-  <dt><code>sisched</code></dt>
-  <dd>Enable LLVM SI Machine Instruction Scheduler.</dd>
   <dt><code>gisel</code></dt>
   <dd>Enable LLVM global instruction selector.</dd>
   <dt><code>w32ge</code></dt>