exceptions: remove unused scp exception.

The code was removed but the exception was not.

Library/Homebrew/exceptions.rb

@@ -512,13 +512,6 @@ class CurlDownloadStrategyError < RuntimeError
   end
 end
 
-# Raised in {ScpDownloadStrategy#fetch}.
-class ScpDownloadStrategyError < RuntimeError
-  def initialize(cause)
-    super "Download failed: #{cause}"
-  end
-end
-
 # Raised by {#safe_system} in `utils.rb`.
 class ErrorDuringExecution < RuntimeError
   attr_reader :cmd