Rm extra indirection by making (color-identifiers:refontify) an alias

For Emacs 25.1 and higher that makes `(color-identifiers:refontify)`
an alias to `(font-lock-flush)`. `(color-identifiers:refontify)` always
was just a wrapper, so no functional change is intended besides
removing the unnecessary indirection.

I haven't found a way to just leave `(if (fboundp … (defalias …)`
expression at the top-level and not cause any byte-compilation errors,
so the solution required a little bit of hackery, but hopefully it
should be safe. At least tests are passing and I haven't found any
problems.

color-identifiers-mode.el

@@ -746,11 +746,11 @@ major mode, identifiers are saved to
     ;; `color-identifiers:get-declarations', which returns all identifiers
     (color-identifiers:get-declarations)))
 
-(defun color-identifiers:refontify ()
-  "Refontify the buffer using font-lock."
-  (if (fboundp 'font-lock-flush)
-      (font-lock-flush)
-    ;; `font-lock-flush' didn't exist prior to Emacs 25.1
+(if (fboundp 'font-lock-flush)
+    (defalias 'color-identifiers:refontify 'font-lock-flush)
+  (declare-function color-identifiers:refontify nil)
+  (defun color-identifiers:refontify ()
+    "Refontify the buffer using font-lock."
     (with-no-warnings
       (and font-lock-mode (font-lock-fontify-buffer)))))