Removed flush_logs_every_n_steps argument from Trainer

[shenoynikhil]

What does this PR do?

Part of #12521

This PR removes the flush_logs_every_n_steps argument in the Trainer Class and the corresponding changes. Made the following edits,

• Removed the documentation from docs/source/common/trainer.rst and docs/source/visualize/logging_advanced.rst
• Removed the should_flush condition in pytorch_lightning/loops/epoch/training_epoch_loop.py
• Removed the flush_logs_every_n_steps attribute from trainer in pytorch_lightning/trainer/connectors/logger_connector/logger_connector.py

resolves #9366

Does your PR introduce any breaking changes? If yes, please list them.

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[awaelchli]

nice, just a minor merge conflict to address!

[shenoynikhil]

nice, just a minor merge conflict to address!

@awaelchli Merged with master to fix the merge conflict