Uninstall OS X Server completely in Yosemite
I got this solution by posting the same on apple discussion forum. I wanted to post the answer here because, it might help someone else. Many thanks to the friend who gave me this solution.

Move the Server application to the Trash. Delete the following items (some may not exist), then reboot:

/.ServerBackups
/var/db/.ServerSetupDone
/Library/Preferences/com.apple.serverd.plist
/Library/Preferences/com.apple.servermgr_accounts.plist
/Library/Preferences/com.apple.servermgr_certs.plist
/Library/Preferences/com.apple.servermgr_info.plist
/Library/Preferences/com.apple.servermgrd.plist
/Library/PrivilegedHelperTools/com.apple.serverd
/Library/Server


Here's Apple Discussion forum link