Remove obsolete variable LIBALKIMIA_INCLUDE_DIR from alkimia package config file

Summary:
In the past alkimia clients needed to use LIBALKIMIA_INCLUDE_DIR for
adding alkimia include pathes. Upgrading to 7.0 makes this obsolete,
because the alkimia package provides an imported cmake target named
'Alkimia::alkimia' which includes the location of alkimia
headers. See README.md for more informations how to use this library.

Test Plan: compiled on linux

Reviewers: tbaumgart

Reviewed By: tbaumgart

Differential Revision: https://phabricator.kde.org/D11286