virgl/ci: drop invalid but overridden empty caching proxy

The variable is undefined, leading to an empty proxy being set, but
luckily it's actually overridden back to the default args, so this was
just dead code.

Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/36955>

src/gallium/drivers/virgl/ci/gitlab-ci.yml

@@ -55,8 +55,6 @@ virgl-traces:
   extends:
     - .virgl-iris-test
     - .virgl-iris-manual-rules
-  variables:
-    PIGLIT_REPLAY_EXTRA_ARGS: "--download-caching-proxy-url=${FDO_HTTP_CACHE_URI}"
 
 .virgl-iris-traces-performance:
   extends:
@@ -66,7 +64,6 @@ virgl-traces:
     HWCI_FREQ_MAX: "true"
     PIGLIT_REPLAY_SUBCOMMAND: profile
     PIGLIT_REPLAY_LOOP_TIMES: 150
-    PIGLIT_REPLAY_EXTRA_ARGS: "--download-caching-proxy-url=${FDO_HTTP_CACHE_URI}"
 
 .android-virgl-llvmpipe:
   extends: