Revert "glsl: Switch to disable-by-default for the GLSL shader cache"

This reverts commit 0f60c6616e.

Piglit and all games tested so far seem to be working without
issue. This change will allow wide user testing and we can decided
before the next release if we need to turn it off again.
Reviewed-by: Marek Olšák <marek.olsak@amd.com>

src/compiler/glsl/tests/cache_test.c

@@ -428,11 +428,6 @@ main(void)
 #ifdef ENABLE_SHADER_CACHE
    int err;
 
-   /* While the shader cache is still experimental, this variable must
-    * be set or the cache does nothing.
-    */
-   setenv("MESA_GLSL_CACHE_ENABLE", "1", 1);
-
    test_disk_cache_create();
 
    test_put_and_get();