chore: remove Custard references to flakybot.yaml (GoogleCloudPlatform#4176)

Follow up to GoogleCloudPlatform#4145, removing Custard config references to flakybot.yaml,
which is no longer in the repository.

Author: briandorsey

.github/config/nodejs-dev.jsonc

@@ -37,7 +37,6 @@
     ".github/PULL_REQUEST_TEMPLATE.md",
     ".github/auto-label.yaml",
     ".github/blunderbuss.yml",
-    ".github/flakybot.yaml",
     ".github/header-checker-lint.yml",
     ".github/scripts/",
     ".github/snippet-bot.yml",

.github/config/nodejs.jsonc

@@ -37,7 +37,6 @@
     ".github/PULL_REQUEST_TEMPLATE.md",
     ".github/auto-label.yaml",
     ".github/blunderbuss.yml",
-    ".github/flakybot.yaml",
     ".github/header-checker-lint.yml",
     ".github/scripts/",
     ".github/snippet-bot.yml",