@GodkRe you do exactly the opposite of how you installed them, they don't come with uninstallers, so the good news is it's easy and doesn't leave registry dust or crap behind. If you don't remember the files involved download the mod again and look at the instructions to install and do the opposite to remove those files.

If they are not in your mods folder they will be in your installation folder, your scripts folder, or your games folder which is not recommended. Sometimes mods will create an ini file rather than include it and many create log files. You can safely delete those. If unsure, do a backup and if you make a mistake verify integrity of your game files.

Exceptions
Once more just look at the instructions from the mod.
Menyo: has additional Menyoo Stuff folder
OpenIV: has an uninstaller
OIV: some have uninstallers, if not you unzip/extract the OIV file (its an archive like a zip, 7z, rar, etc) and look for file called assembly.xml. it will have the information regarding which files were placed where.

In your case:
Installation (Manual):

Open .OIV with rar/zip program and get the files Create folder called naturalmotion to
GTAV/ update/ update.rpf/ common/data
and place behaviours.xml there Copy Physicstasks.ymt to:
GTAV/ update/ update.rpf/ x64/ data/ tune

IF you still don't get these reactions to happen, install the Physicstasks.ymt to x64a.rpf/data/tune and
Behaviours.xml to common.rpf/data/naturalmotion/ so that the game definetely reads them

Uninstallation:
-Remove the file from tune folder
-Remove "naturalmotion" folder