Problem

You want to take a screenshots without drop shadows.

Solutions

There are several methods:

Method 1

Press and hold the Option key ⌥ while taking screenshot.

Method 2

Disable the drop shadow globally

Run this command in the Terminal

defaults write com.apple.screencapture disable-shadow -bool true && killall SystemUIServer

If you want to re-enable the drop shadow, run this command in the Terminal

defaults write com.apple.screencapture disable-shadow -bool false && killall SystemUIServer