softpipe: Drop the use_tgsi debug flag.

We lower to TGSI inside the driver just fine, no need to ask the frontend
for it.

Reviewed-by: Alyssa Rosenzweig <alyssa@rosenzweig.io>
Acked-by: Marek Olšák <marek.olsak@amd.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/23114>

docs/envvars.rst

@@ -992,9 +992,6 @@ Softpipe driver environment variables
    ``use_llvm``
       the Softpipe driver will try to use LLVM JIT for vertex
       shading processing.
-   ``use_tgsi``
-      if set, the Softpipe driver will ask to directly consume TGSI, instead
-      of NIR.
 
 LLVMpipe driver environment variables
 -------------------------------------