There is no deduction or other tax related issues when you pay back your loan. 

When you take a loan out from a 401(k) plan, the money you take out is tax-exempt as long as you pay it back on time. As you make payments, the interest you pay is paid to your 401(k) account. Your payments are not deducted on your tax return nor the loan. 

Refer to the TurboTax article What happens if I have a 401(k) loan but later lose or quit my job? if you stop working at your job. 

Refer to the IRS Topic Considering a loan from your 401(k) plan? for more information.

I saw a good explanation of this by @dmertz .  When you received the loan, it was tax-free.  You are already repaying the loan from those tax-free dollars, so there is no further tax deduction or reduction.  The interest is coming from your after-tax money, and it is not deductible, but it does go to your credit in the account rather than to a bank, so it benefits you in the long run. 

As Opus 17 said, your repayments of the loan principal are just putting back the pre-tax money that you took out.  How you move money around outside of the 401(k) doesn't change that in any way.  (Your pay is money outside of the 401(k).)