Fix VK_ICD_FILENAMES not being set correctly

-We don't need to validate the file paths, there are two reasons this is incorrect:

(1) The file paths are already verified when they are auto-added here:

089b865290/lutris/sysoptions.py (L72)

(2) Since the files are concatenated here:

089b865290/lutris/sysoptions.py (L78)

it makes system.path_exists(vk_icd) invalid, because the value of vk_icd is now path1:path2:path3:path4 and so on, instead of a single file path.

Therefore the check for system.path_exists(vk_icd) fails and VK_ICD_FILENAMES never actually gets set by Lutris.

lutris/runners/runner.py

@@ -177,7 +177,7 @@ class Runner:  # pylint: disable=too-many-public-methods
 
         # Vulkan ICD files
         vk_icd = self.system_config.get("vk_icd")
-        if vk_icd and vk_icd != "off" and system.path_exists(vk_icd):
+        if vk_icd:
             env["VK_ICD_FILENAMES"] = vk_icd
 
         runtime_ld_library_path = None