gnu: weechat: Update to 2.6.

* gnu/packages/irc.scm (weechat): Update to 2.6.
[source]: Remove obsolete patch.
[inputs]: Replace python-2 with python(@3).
[arguments]: Remove ‘wrap’ phase.
* gnu/packages/patches/weechat-python.patch: Delete file.
* gnu/local.mk (dist_patch_DATA): Remove it.