Remove unused isinstance() in backends.py

This dead code was made impossible in 251dff5.

mesonbuild/backend/backends.py

@@ -1831,8 +1831,6 @@ def generate_header_install(self, d: InstallData) -> None:
                     outdir_name = os.path.join('{includedir}', subdir)
 
             for f in h.get_sources():
-                if not isinstance(f, File):
-                    raise MesonException(f'Invalid header type {f!r} can\'t be installed')
                 abspath = f.absolute_path(srcdir, builddir)
                 i = InstallDataBase(abspath, outdir, outdir_name, h.get_custom_install_mode(), h.subproject, tag='devel', follow_symlinks=h.follow_symlinks)
                 d.headers.append(i)