RFC: Remove OMF Support from DMD

[Adam Wilson]

Hello Dlang,

We are looking for feedback on this PR to begin the OMF removal 
process: https://github.com/dlang/dmd/pull/16422

OMF was deprecated back in January of 2022 and it's time to start 
the removal process, however before we begin, we are looking for 
feedback from anybody who might still be using OMF in your code.

For background OMF deprecation is part of a long-running goal of 
cleaning up and simplifying the DMD code-base, so while the 
long-term goal is to drop OMF, we need to know if any users need 
any additional time to transition their projects.