chore: remove .opacity() mixin as it is supported cross-browser

less/account.less

@@ -77,7 +77,7 @@
 
 		&:hover {
 			.controls {
-				.opacity(0.8);
+				opacity: 0.8;
 			}
 		}
 
@@ -85,7 +85,7 @@
 			text-align: center;
 			height: 200px;
 			line-height: 200px;
-			.opacity(0);
+			opacity: 0;
 			.transition(opacity .15s linear);
 			cursor: pointer;
 			pointer-events: none;

less/category.less

@@ -105,7 +105,7 @@
 						border: 1px solid @btn-success-border;
 						color: @btn-success-color;
 					}
-				} 
+				}
 			}
 
 			.fa-stack {

less/groups.less

@@ -29,7 +29,7 @@
 
 		&:hover {
 			.controls {
-				.opacity(0.8);
+				opacity: 0.8;
 			}
 		}
 
@@ -37,7 +37,7 @@
 			text-align: center;
 			min-height: 200px;
 			line-height: 200px;
-			.opacity(0);
+			opacity: 0;
 			.transition(opacity .15s linear);
 			cursor: pointer;
 			pointer-events: none;

less/mixins.less

@@ -27,13 +27,6 @@
 	}
 }
 
-.opacity(@opacity: 1) {
-	-moz-opacity: @opacity;
-	opacity: @opacity;
-	-ms-filter: ~`"progid:DXImageTransform.Microsoft.Alpha(opacity=(" + "@{opacity}" * 100 + "))"`;
-	filter: ~`"alpha(opacity = (" + "@{opacity}" * 100 + "))"`;
-}
-
 .border-radius (@radius: 5px) {
 	-webkit-border-radius: @radius;
 	-moz-border-radius: @radius;

less/topic.less

@@ -413,7 +413,7 @@
 
 .topic {
 	&.deleted {
-		.opacity(0.3);
+		opacity: 0.3;
 
 		.votes {
 			display: none;
@@ -438,7 +438,7 @@
 
 			&.deleted {
 				> .content {
-					.opacity(0.3);
+					opacity: 0.3;
 				}
 
 				.votes {