[Mellanox]Removing FEC from port_config.ini for SN5600 SKUs (#21403)

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#### Why I did it
Removed FEC mode from port_config.ini for SN5600 SKUs. FEC mode needs to come as part of user configuration as it depends on peer end capabilities as well.

##### Work item tracking
- Microsoft ADO **(number only)**:

#### How I did it
Modified port_config.ini

#### How to verify it
Loading the SKU and testing default configuration generation

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- [ ] 201911
- [ ] 202006
- [ ] 202012
- [ ] 202106
- [ ] 202111
- [ ] 202205
- [ ] 202211
- [ ] 202305

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