glsl: drop cache_fallback

This turned out to be a dead end, it is much easier and less error
prone to just cache the IR used by the drivers backend e.g. TGSI or
NIR.

Cc: "17.2 17.3" <mesa-stable@lists.freedesktop.org>
Reviewed-by: Tapani Pälli <tapani.palli@intel.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>

src/compiler/glsl/shader_cache.cpp

@@ -1299,7 +1299,7 @@ shader_cache_read_program_metadata(struct gl_context *ctx,
       return false;
 
    struct disk_cache *cache = ctx->Cache;
-   if (!cache || prog->data->cache_fallback || prog->data->skip_cache)
+   if (!cache || prog->data->skip_cache)
       return false;
 
    /* Include bindings when creating sha1. These bindings change the resulting