ci/piglit: avoid copying /piglit across filesystems when unused

This `mv` copies the folder from the read-only docker image to the
run time overlayfs, which is expensive and unnecessary, and slow on
devices using network disks.

`rm` by comparison is instant, and there is no reason to keep the folder
around if we're taking it away from the expected path anyway.

Fixes: 8d08cde667 ("ci/piglit: Use structured tagging for Piglit")
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/35500>

.gitlab-ci/deqp-runner.sh

@@ -35,10 +35,10 @@ if [ -n "$PIGLIT_TAG" ]; then
   # Are we using the right Piglit version?
   ci_tag_test_time_check "PIGLIT_TAG"
 elif [ -d "/piglit" ]; then
-  # The job does not inherit from .test-piglit, so we move it out of the way.
+  # The job does not inherit from .test-piglit, so we remove it.
   # This makes sure that we can both do the right version checks when needed,
   # and also optimise our dependencies so we don't pull unneeded stuff.
-  mv /piglit /piglit.unused
+  rm -r /piglit
 fi
 
 # Ensure Mesa Shader Cache resides on tmpfs.