Remove 'vectorizers' label from PR labeler (#67810)

It's subsumed by an order of magnitude more popular `vectorization` label that is applied for the same path patterns.

Statistics (issues and PRs together):
`vectorization`: 91 open, 91 closed
`vectorizers`: 8 open, 5 closed 

All `vectorizers` usages has occurred in just the past 2 weeks, and likely by our bot.