doc: Drop remaining package_ebuild_eapi_4_slot_abi

Those where dropped in d34f2dc ("Remove support for 4-slot-abi
EAPI"), but this one wasn't removed causing doc build failures.

Fixes: d34f2dc
Signed-off-by: Florian Schmaus <flow@gentoo.org>
Signed-off-by: Ulrich Müller <ulm@gentoo.org>

doc/package/ebuild.docbook

@@ -9,7 +9,6 @@
 &package_ebuild_eapi_2;
 &package_ebuild_eapi_3;
 &package_ebuild_eapi_4;
-&package_ebuild_eapi_4_slot_abi;
 &package_ebuild_eapi_5;
 </section>
 </chapter>