xivo-yealink: remove 2015-03 and 25.73.0.20 plugins

The xivo-yealink-25.73.0.20 plugin is replaced by the xivo-yealink-v73
plugin.

The xivo-yealink-2015-03 plugin is replaced by
* the xivo-yealink-v80 (for T4X)
* the xivo-yealink-v73 (for most T2X and the W52P)
* the xivo-yealink-72.0 (for T19P and T21P)
* the xivo-yealink-70.0 (for T3X)