BUG: set df.plot.area linewidth to 0

- [x] closes #32106 
- [ ] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry (`1.1.0`)

Sure, however tests as they are don't pass on my machine: 7 of them fail and I can't really make sense of the logs, even on upstream master 8 won't pass.
I included them if you want to help me have a look: [pandas_test.log](https://github.com/pandas-dev/pandas/files/4266534/pandas_test.log). @leandermaben Can you open an issue for the failing tests? @leogermond not sure about your local environment but can you still add one here? Can leverage CI Closing as stale, but happy to reopen if you want to address comments and provide a test