Prevent +moveup & +movedown from effecting ground movement

[braem]

Closes a card for 0.8.7

m_flUpMove is what comes from +moveup and +movedown so just cut that out of the speed.

Checklist

• I have thoroughly tested all of the code I have modified/added/removed to ensure something else did not break
• If there is a localization token change, I have updated the momentum_english_ref.res file with the changes, ran tokenizer.py to generate an up-to-date localization file, and have committed both the .res file changes and the new localization .txt file
• If I introduced new h/cpp files, I have added them to the appropriate project's VPC file (server_momentum.vpc / client_momentum.vpc / etc)
• If I have added or modified any visual assets (models, materials, panels, effects, etc), I have taken screenshots / videos of them and attached them to this PR directly (screenshots uploaded through github, videos uploaded to youtube and linked)
• If I have modified any console command, console variable, or momentum entity, I have opened an issue (or a PR) for it in the Momentum Mod documentation repository
• My commits are relatively small and scoped to the best of my ability
• My branch has a clear history of changes that can be easy to follow when being reviewed commit-by-commit
• My branch is functionally complete; the only changes to be done will be those potentially requested in code review