info.xml - Don't officially list 4.6 compatibility

To run on CiviCRM v4.6, you need to specifically use v4.6.26+ along with the
patches from [PR #9555](civicrm/civicrm-core#9555).
Although I do expect some folks will use the patches, it doesn't feel
accurate to list it for general consumption on 4.6 sites.

Note: You can enable/use extensions regardless of what's in the
`<compatibility>` / `<ver>` tags.  The tag's main purpose is to filter the
list of extensions advertised in the public directory.

info.xml

@@ -18,7 +18,6 @@
   <version>1.0-beta1</version>
   <develStage>beta</develStage>
   <compatibility>
-    <ver>4.6</ver>
     <ver>4.7</ver>
   </compatibility>
   <requires>