Can I remove the navigation bar as this is distracting and clutters the site especially on Stack Overflow.
Go to your profile page.
Click Edit Profile & Settings.
Click "Preferences" under "Site Settings".
Tick the "Hide left navigation" checkbox:
Profit. (No need to click any save button, action is immediate)
Note that this is a per-site setting, you'll have to repeat this on every site in Stack Exchange you join.
Also, hiding the left nav does not restore the previous tabbed design. Instead, it will collapse all the links into hamburger menu on the top left, as if you're always on mobile: