Why unable to find the exact path for grains storage in master?
These are built-in grains, which are collected at minion startup. Here's the code responsible for generating them.

You cannot remove them, but you can overwrite them, for example to an empty value in /etc/salt/grains:

osfinger: null


or with grains execution module: salt minion1 grains.delval osfinger.

Keep in mind that some of the core grains are essential for the correct functioning of salt, e.g. module providers for pkg module rely on them.