Clean up unnecessary flint include directories #21803
Description
There is no reason to add $SAGE_LOCAL/include/flint as include directory.
CC: @jdemeyer
Component: cython
Author: Jeroen Demeyer
Branch/Commit: 5306746
Reviewer: Marc Mezzarobba, Matthias Koeppe
Issue created by migration from https://trac.sagemath.org/ticket/21803