Add the following to ~/.gtkrc-2.0:

 binding "gtk-clear-text-entry" {
   bind "<ctrl>k" {
      "delete-from-cursor" (paragraph-ends, 1)
   } 
   bind "<ctrl>u" {
      "move-cursor" (paragraph-ends, -1, 0) 
      "delete-from-cursor" (paragraph-ends, 1)
   }
 } 
 class "GtkEntry" binding "gtk-clear-text-entry"
 class "GtkTextView" binding "gtk-clear-text-entry"

Note that this method is better than just inserting in ~/.gtkrc-2.0:

 gtk-key-theme-name = "Emacs"

because the latter disables the usage of Ctrl-W to close tabs.