Changed the argument of not_found for 'unprotect'

not_found appends string "Not Found" to the argument causing the resulting message to be "Branch does not exist Not Found" which is an incorrect error message.

Changed the argument of not_found! for 'unprotect'  command to "Branch" from "Branch does not exist".  This makes the final error message to appear as "Branch Not Found" which is correct and same as error messages for other commands like 'protect'.

lib/api/branches.rb

@@ -64,7 +64,7 @@ module API
         authorize_admin_project
         @branch = user_project.repository.find_branch(params[:branch])
-        not_found!("Branch does not exist") unless @branch
+        not_found!("Branch") unless @branch
         protected_branch = user_project.protected_branches.find_by(name: @branch.name)
         protected_branch.destroy if protected_branch