[dev] Remove the beta channel from the list

dev.yml is used to test against the latest stable version of the
beat packages. The CI build system now uses the new strict channel
policy of conda which means that it does not search further after a
package is found in a channel. Thus if the specified version is not
found, it ends up with an error.

dev.yml

 name: beat-web
 channels:
-  - http://www.idiap.ch/software/beat/conda/label/beta
   - http://www.idiap.ch/software/beat/conda
   - defaults
 dependencies: