Fix defining a few faces as empty

e72afa9dbf replaced nil, which is invalid as a face definition,
with ((t (:inherit default))), but that does not actually result in
an "empty" face, which doesn't set any attributes.  Instead it doubles
down on all attributes set for the default face.  Replace that with
((t)), which accomplishes what appears to have been the intention.

* lisp/net/shr.el (shr-h4, shr-h5, shr-h6):
* lisp/nxml/nxml-mode.el (nxml-text):
* lisp/progmodes/make-mode.el (makefile-shell): Use empty face
definition ((t)), instead of ((t (:inherit default))).

lisp/net/shr.el

@@ -292,17 +292,17 @@ temporarily blinks with this face."
   :version "28.1")
 
 (defface shr-h4
-  '((t (:inherit default)))
+  '((t))
   "Face for <h4> elements."
   :version "28.1")
 
 (defface shr-h5
-  '((t (:inherit default)))
+  '((t))
   "Face for <h5> elements."
   :version "28.1")
 
 (defface shr-h6
-  '((t (:inherit default)))
+  '((t))
   "Face for <h6> elements."
   :version "28.1")
 

lisp/nxml/nxml-mode.el

@@ -152,7 +152,7 @@ This is not used directly, but only via inheritance by other faces."
   :group 'nxml-faces)
 
 (defface nxml-text
-  '((t (:inherit default)))
+  '((t))
   "Face used to highlight text."
   :group 'nxml-faces)
 

lisp/progmodes/make-mode.el

@@ -93,7 +93,7 @@
   :version "22.1")
 
 (defface makefile-shell
-  '((t (:inherit default)))
+  '((t))
   ;;'((((class color) (min-colors 88) (background light)) (:background  "seashell1"))
   ;;  (((class color) (min-colors 88) (background dark)) (:background  "seashell4")))
   "Face to use for additionally highlighting Shell commands in Font-Lock mode."