Use same colors for disabled buttons

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

app/assets/stylesheets/generic/buttons.scss

@@ -67,8 +67,6 @@
     &.disabled,
     &[disabled] {
       color: #ffffff;
-      background-color: #3286b1;
-      border-color: #286e8e;
     }
   }
 
@@ -83,8 +81,6 @@
     &.disabled,
     &[disabled] {
       color: #ffffff;
-      background-color: #47a447;
-      border-color: #398439;
     }
   }
 
@@ -99,8 +95,6 @@
     &.disabled,
     &[disabled] {
       color: #ffffff;
-      background-color: #d2322d;
-      border-color: #ac2925;
     }
   }
 
@@ -115,8 +109,6 @@
     &.disabled,
     &[disabled] {
       color: #ffffff;
-      background-color: #e38c02;
-      border-color: #e38c02;
     }
   }