Bug 1356476 - Don't enable tooltool cache on web platform tests. r=mshal

Those jobs, running on taskcluster, don't have a persistent cache
anyways, so we might as well not pretend setting a tooltool cache does
anything, especially considering the configured directory is not
writeable anyways.

--HG--
extra : rebase_source : a435bce42b4c181a6690a42e068bb2e0e875d5e8

testing/mozharness/configs/web_platform_tests/prod_config.py

@@ -40,8 +40,5 @@ config = {
     "download_minidump_stackwalk": True,
 
     "download_tooltool": True,
-
-    "tooltool_cache": "/builds/tooltool_cache",
-
 }