[FPGA] Remove mandatory DEVICE_FLAG from mem_channel tutorial

[justin-rosner]

Description

This change removes the fatal error when the user does not specify the DEVICE_FLAG when compiling. The motivation behind this change is that even when users specify the correct DEVICE_FLAG, they will get a cmake warning telling them that DEVICE_FLAG was unused.

External Dependencies

• N/A

Type of change

Please delete options that are not relevant. Add a 'X' to the one that is applicable.

• [X] Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Implement fixes for ONSAM Jiras

How Has This Been Tested?

Manually compiled the design to reports to make sure that the cmake warning is no longer displayed to users.