[REF] qweb: drop support of #{.} string interpolation

We only support the {{.}} syntax.

Part of #128

doc/component.md

@@ -379,7 +379,7 @@ The `t-widget` directive also accepts dynamic values with string interpolation
 
 ```xml
 <div t-name="ParentWidget">
-    <t t-widget="ChildWidget#{id}"/>
+    <t t-widget="ChildWidget{{id}}"/>
 </div>
 ```
 
@@ -390,14 +390,6 @@ class ParentWidget {
 }
 ```
 
-Similarly to `t-attf-`, there is an alternate form of string interpolation:
-
-```xml
-<div t-name="ParentWidget">
-    <t t-widget="ChildWidget{{id}}"/>
-</div>
-```
-
 **CSS and style:** there is some specific support to allow the parent to declare
 additional css classes or style for the sub widget: css declared in `class`, `style`, `t-att-class` or `t-att-style` will be added to the
 root widget element.
@@ -754,20 +746,13 @@ The `t-ref` directive also accepts dynamic values with string interpolation
 `id` set to 44 in the rendering context,
 
 ```xml
-<div t-ref="widget_#{id}"/>
+<div t-ref="widget_{{id}}"/>
 ```
 
 ```js
 this.refs.widget_44;
 ```
 
-Similarly to `t-attf-` and `t-widget`, there is an alternate form of string
-interpolation:
-
-```xml
-<div t-ref="widget_{{id}}"/>
-```
-
 ### Asynchronous rendering
 
 Working with asynchronous code always adds a lot of complexity to a system. Whenever