Remove redundant name check when registering a plugin

[svvac]

name is equal to plugin_spec.as when the latter is set by get_plugin_short_name(). The conflict check against the alias is thus redundant.

Dropping this allows plugins to require aliased plugins, see #1017.

[svvac]

Or, if you prefer, guarding the second check as demonstrated in eae47319af7f3807390b801e3d5d7d8dba6503b2.