Don't create text-buttons unnecessarily, just bind RET and SPC in the
diagnostics buffer to a command that figures out which diagnostic it
was invoked on.
* lisp/progmodes/flymake.el
(flymake--diagnostics-buffer-mode-keymap): Renamed from
flymake--diagnostics-buffer-button-keymap.
(flymake-show-diagnostic-at-point): Don't take a button.
(flymake-goto-diagnostic-at-point): Don't pass button to
flymake-show-diagnostic-at-point.
(flymake--diagnostics-buffer-entries): Simplify.