Remove toolcache decorator

This decorator enabled us to use the functionality of the Actions
toolcache within the runner too.
Now that we've deleted the runner we no longer need it.

src/actions-util.ts

@@ -57,13 +57,6 @@ export function getTemporaryDirectory(): string {
     : getRequiredEnvParam("RUNNER_TEMP");
 }
 
-export function getToolCacheDirectory(): string {
-  const value = process.env["CODEQL_ACTION_TOOL_CACHE"];
-  return value !== undefined && value !== ""
-    ? value
-    : getRequiredEnvParam("RUNNER_TOOL_CACHE");
-}
-
 /**
  * Gets the SHA of the commit that is currently checked out.
  */