mail-client/trojita: Unconditionally enable the abook plugin

mail-client/trojita: Unconditionally enable the abook plugin

Trojita supports different implementations of an "address book" via
plugins that are loaded at runtime. Only one plugin can be active at a
time, but any number of them can be built and installed. The user can
pick and select an appropriate plugin via a GUI configuration.

There is no need to disable the "abook" plugin, and upstream expects
this plugin to be always available. There are no external dependencies,
and the `abook` binary is not used.

Fixes: e0e382d4f9341 (mail-client/trojita: Add USE addressbook)
Closes: https://bugs.gentoo.org/692214
Signed-off-by: Jan Kundrát <jkt@kde.org>
Closes: https://github.com/gentoo/gentoo/pull/12794
Signed-off-by: Andreas Sturmlechner <asturm@gentoo.org>