style(Renovate): Remove Yarn packageNameTemplate

Regex managers use the captured `depName`, in this case `yarn`, from the
match string when no package name is specified. Hence, there is no need
to specify explicitly that the package name is `yarn`.

default.json

@@ -147,7 +147,6 @@
       "customType": "regex",
       "fileMatch": ["^\\.tool-versions$"],
       "matchStrings": ["(?<depName>yarn)\\s+(?<currentValue>(\\d+\\.){2}\\d+)"],
-      "packageNameTemplate": "yarn",
       "datasourceTemplate": "npm",
       "depTypeTemplate": "packageManager",
       "extractVersionTemplate": "^v(?<version>.*)$"