Disabled checked states wrong background

[ffoodd]

Fixes #29809 for v4.

V5 doesn't need this since :disabled simply decreases opacity for disabled fields, whatever their background is.

[XhmikosR]

@ffoodd it seems this results in duplicate background-color properties:

 .custom-checkbox .custom-control-input:disabled:checked ~ .custom-control-label::before {
   background-color: rgba(0, 123, 255, 0.5);
+  background-color: rgba(0, 123, 255, 0.5);
 }