[urls to hub] Replace outdated model tags with their now-canonical pipeline types
> ### [Internal tracker 🌔 ](https://github.com/huggingface/moon-landing/pull/1558)

#### Context:

On the hub side, we are using the metadata generated by transformers (hat/tip @sgugger) to replace the pattern matching, BUT I'm also proposing to take this opportunity to remove the `lm-head`, `seq2seq`, `causal-lm`, `masked-lm` auto-generated tags (it's still possible to add them manually to a model card, of course)

(they are superseded by the pipeline tag aka. what we call the Task on hf.co/models)

This might break some stuff though, which is why your feedback will be important. 
also cc'ing @osanseviero for info before merging!