Use inherit on alert-heading color

Heading then inherits from the parent alert.
Avoids a lot of extra unneccessary CSS.

less/alerts.less

@@ -9,11 +9,11 @@
   background-color: @warningBackground;
   border: 1px solid @warningBorder;
   .border-radius(4px);
-}
-.alert,
-.alert-heading {
   color: @warningText;
 }
+.alert-heading {
+  color: inherit;
+}
 
 // Adjust close link position
 .alert .close {
@@ -29,32 +29,20 @@
 .alert-success {
   background-color: @successBackground;
   border-color: @successBorder;  
-}
-.alert-success,
-.alert-success .alert-heading {
   color: @successText;
 }
 .alert-danger,
 .alert-error {
   background-color: @errorBackground;
   border-color: @errorBorder;
-}
-.alert-danger,
-.alert-error,
-.alert-danger .alert-heading,
-.alert-error .alert-heading {
   color: @errorText;
 }
 .alert-info {
   background-color: @infoBackground;
   border-color: @infoBorder;
-}
-.alert-info,
-.alert-info .alert-heading {
   color: @infoText;
 }
 
-
 // Block alerts
 // ------------------------
 .alert-block {