git-gui: stop including GIT-VERSION-FILE file

git-gui: stop including GIT-VERSION-FILE file

The "GITGUI_VERSION" variable is made available by generating and
including the "GIT-VERSION-FILE" file. Its value has been used in
various build steps, but in the preceding commits we have refactored
those to instead source the "GIT-VERSION-FILE" directly. As a result,
the variable is now only used in a single recipe, and this use can be
trivially replaced with sed(1).

Refactor the recipe to do so and stop including "GIT-VERSION-FILE" to
simplify the build process.

Signed-off-by: Patrick Steinhardt <ps@pks.im>