Funny thing is, that I've worked with multiple (unrelated) teams who simply refuse to use branches. This keeps boggling my mind (I usually don't work with code directly, since I am a tester, and focus on the end to end verification and validation, not with the details and unit tests), especially the argument that is used: "we want to move fast". On one occasion I could understand it, since they used SVN and externals, and each branching process somehow required 3 developers and 4-8 hours (I never bothered to drill down to the "why" behind those numbers, but I believe they mismanaged the sources, and used externals, where they should have been using just binary dependencies). Yet my current team uses git, and straight on decided that branching is too much hassle.
The idea is you create a branch to delete those conditionals before the feature flag is activated. Once it's activated, you merge in that branch.