Remove text saying low processor mode only works on desktop

[skyace65]

I can't go through the engine code to confirm this (I don't know C++), but there's no mention of it only working on desktop in the description for low processor mode for the OS class, which is the same setting. And I know because #59607 exists that it at least works on Andoird.

I'll gladly update this PR if this isn't the case though and it still doesn't work on something. Closes https://github.com/godotengine/godot-docs/issues/9021

[clayjohn]

Cc @m4gr3d

[akien-mga]

Thanks!