How to remove Windows Operating system from Bootcamp?
Apple helps:


  Boot Camp: Remove Windows from your Mac  
  
  How you remove Windows from your Mac depends on whether you installed Windows on a disk that has multiple partitions or one that has a single partition.
  
  Important: Back up all important data stored on your Windows partition. 
  
  If you installed Windows on a disk that has multiple partitions
  
  
  Start up your Mac in OS X.
  Quit all open apps and log out any other users.
  Open Boot Camp Assistant, then click Continue.
  Select “Remove Windows 7 or later version,” then click Continue.
  Do one of the following:
  
  If your Mac has a single internal disk, click Restore.
  If your Mac has multiple internal disks, select the Windows disk, select “Restore disk to a single OS X partition,” then click Continue.
  
  
  
  If you installed Windows on a disk that has a single partition
  
  
  Start up your Mac in OS X.
  Open Disk Utility, located in the Other folder in Launchpad.
  Select the Windows disk, click Erase, choose the Mac OS Extended (Journaled) >format, then click the Erase button.