Community Builder (CB) is free (advanced paid version is also there) social extension. This extension shows a link in footer "Powered by Community Builder".
You can buy paid version of CB and get this footer copyright link removed. However, in case you want to remove from the code, follow these steps. I have tested on Joomla 3.4.x version with CB 2.0.x version.
Using FTP or cPanel File Manager, open following file from your Joomla installation:
Find (use ctrl + F) the function cbPoweredBy. This function is responsible for displaying footer. It is located almost at the end of file.
In the above function, change the following line:
Save the file. Refresh the front-end browser page. The footer should have gone!