Factorize a bit the checkbox-related CSS

Since the `border: 0;` property is applied
to every checkbox, there is no need for a class.
This is also a good opportunity to use ternaries
for the `checked` attribute, instead of clumsy `if`.

airsonic-main/src/main/webapp/style/default-without-mediaelement.css

@@ -204,6 +204,10 @@ input[type=submit]:active, input[type=button]:active {
     background: #e9e9e9;
 }
 
+input[type=checkbox] {
+	border: 0;
+}
+
 input:hover, textarea:hover, select:hover {
     border: 1px solid darkgray;
 }
@@ -267,6 +271,10 @@ img {
     vertical-align: middle;
 }
 
+.monospace {
+	font-family: monospace;
+}
+
 .dropshadow {
     box-shadow: 2px 2px 10px #333333;
 }
@@ -398,10 +406,6 @@ img {
     line-height: 1em;
 }
 
-.checkbox {
-    border: 0
-}
-
 /* Table with simple lines between the cells. */
 .ruleTable {
     border-collapse: collapse;