Fix some minor LESS syntax errors (handled by lessc, but important if we switch to a less permissive LESS compiler)

wagtail/wagtailadmin/static/wagtailadmin/css/components/formatters.less

@@ -122,7 +122,7 @@
     -webkit-font-smoothing: auto;
     font-size:0.80em;
     margin:0 0.5em;
-    background:white url(~"@{static-root}bg-dark-diag.svg");
+    background:white url("@{static-root}bg-dark-diag.svg");
     
     &.primary{
         color:@color-grey-2;

wagtail/wagtailadmin/static/wagtailadmin/css/components/icons.less

@@ -12,7 +12,7 @@
 @media screen and (-webkit-min-device-pixel-ratio:0) {
     @font-face {
         font-family: 'wagtail';
-        src: url("@{css-root}fonts/wagtail.svg#wagtail") format("svg"),
+        src: url("@{css-root}fonts/wagtail.svg#wagtail") format("svg");
     }
 }
 
@@ -60,7 +60,7 @@
 }
 
 
-.icon-cogs:before, {
+.icon-cogs:before {
     content: "a";
 }
 .icon-doc-empty-inverse:before {

wagtail/wagtailadmin/static/wagtailadmin/css/layouts/page-editor.less

@@ -6,7 +6,7 @@
 }
 
 .objects{
-    background:url(~"@{static-root}bg-dark-diag.svg");
+    background:url("@{static-root}bg-dark-diag.svg");
 }
 .object{  
     position:relative;