kbuild: remove partial section mismatch detection for built-in.a

When built-in.o was incrementally linked with 'ld -r', the section
mismatch analysis for the individual built-in.o was possible when
CONFIG_DEBUG_SECTION_MISMATCH was enabled.

With the migration to the thin archive, built-in.a (former, built-in.o)
is no longer an ELF file.  So, the modpost does nothing useful.
scripts/mod/modpost.c just checks the header to bail out, as follows:

        /* Is this a valid ELF file? */
        if ((hdr->e_ident[EI_MAG0] != ELFMAG0) ||
            (hdr->e_ident[EI_MAG1] != ELFMAG1) ||
            (hdr->e_ident[EI_MAG2] != ELFMAG2) ||
            (hdr->e_ident[EI_MAG3] != ELFMAG3)) {
                /* Not an ELF file - silently ignore it */
                return 0;
        }

We have the full analysis in the final link stage anyway, so we would
not miss the section mismatching.

I do not see a good reason to require extra linking only for the
purpose of the per-directory analysis.  Just get rid of this part.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>