use baseBorderRadius instead of corner_radius

as it already exist in Bootstrap

IPython/html/static/base/less/mixins.less

@@ -7,5 +7,5 b''
 }
 
 .corner-all {
-    border-radius: @corner_radius;
-}
  No newline at end of file
+    border-radius: @baseBorderRadius;
+}

IPython/html/static/base/less/variables.less

@@ -7,5 +7,4 b''
 
 // Our own global variables for all pages go here
 
-@corner_radius:         4px;
 @code_line_height:      1.231em;

IPython/html/static/notebook/less/codemirror.less

@@ -45,8 +45,8 b''
 .CodeMirror-gutters {
   // This is needed because our cell has rounded corners, otherwise the gutter area square
   // corner cuts into the rounded cell border.
-  border-bottom-left-radius: @corner_radius;
-  border-top-left-radius: @corner_radius;
+  border-bottom-left-radius: @baseBorderRadius;
+  border-top-left-radius: @baseBorderRadius;
 }
 
 .CodeMirror pre {

IPython/html/static/notebook/less/notificationwidget.less

@@ -4,7 +4,7 b''
     margin: 2px 4px;
     z-index: 10;
     border: 1px solid #ccc;
-    border-radius: @corner_radius;
+    border-radius: @baseBorderRadius;
     background: rgba(240, 240, 240, 0.5);
     
 }