[FIX] web: simplify the datepicker template

The library needs to have a 'relative' positioned parent to place
the datetimepicker correctly.
We don't do that in the template
anymore because it was not working in all the cases.
The template which instantiates the datepicker should define
a parent in position 'relative'.

addons/web/static/src/less/datepicker.less

@@ -12,13 +12,7 @@
   border: 0;
 }
 
-.o_bootstrap_datepicker_parent{
-    position: relative;
-}
-
 .o_datepicker {
-    position: relative;
-
     .o_datepicker_input {
         width: 100%;
     }

addons/web/static/src/xml/base.xml

@@ -831,17 +831,13 @@
 
 <t t-name="web.datepicker">
     <t t-set="placeholder" t-value="widget.getParent().node and widget.getParent().node.attrs.placeholder"/>
-    <div>
-        <div class="o_bootstrap_datepicker_parent"></div>
-        <div class="o_datepicker">
-            <input type="text"
-                class="o_datepicker_input"
-                t-att-name="widget.name"
-                t-att-placeholder="placeholder"/>
-            <span class="o_datepicker_button"/>
-        </div>
+    <div class="o_datepicker">
+        <input type="text"
+            class="o_datepicker_input"
+            t-att-name="widget.name"
+            t-att-placeholder="placeholder"/>
+        <span class="o_datepicker_button"/>
     </div>
-
 </t>
 <t t-name="FieldSelection">
     <span t-if="widget.get('effective_readonly')"/>