Question, what do you mean by unnecessary? Could you give a few examples?
Mmm.
Does it ever happen to you that your character just keeps fixated on something?
Like you want to write that a character likes to paint.
You can do it,
"I like to paint."
Or
"I like to paint. It helps me express myself. Express emotions I can't with words."
But anything more than that kind of dwells in the unnecessary part.
Like, you want to show that your character is really obsessed with painting, you can write, "and then she continued to ramble about the colour theory, and great artists like Van Gough."
But don't actually start writing long paragraphs describing colour theory or other paintings(unless they are related to plot) as that would just exhaust the readers.
Same if you want to write an MC who is easily distracted. Kind of like ADHD situation.
And currently he is preparing for his exams.
You can write,
"I couldn't focus on the words, my mind drifted elsewhere. It was more focused on the squirrels outside, sometimes the tiles on the floor."
But don't write something like,
"My eyes wandered to a squirrel on the oak tree in front of my house. It had a brown bushy tail. It ran as soon it noticed my glanced and my eyes followed it. Now they were looking at cars. I never noticed how much smoke a car releases. I wonder if it is running on petrol or diesel. Legally cars aren't allowed to run on diesel in this area but i wouldn't be surprised if someone just broke that law. It also could be that his engine or exhaust pipe is blocked. Just besides the running car were houses, they were of a nice colour. A nice shade of Yellow. Though some were old enough to have their paint fall off. I took a look at my room to compare and the contrast sure was visible. My room looks very nice as well, the blue walls have a good synergy with white tiles."
So, while the second paragraph may look attractive, because you may think it is more descriptive and allows the readers to form a better image, it is just so tiresome for both you and your audience.
Like, if your story doesn't need traffic facts, nor does MC's knowledge of what colours look nice ever comes useful: then just don't write this unnecessary stuff.