Fix pdf build failure with texinfo 6.1.0

Fix pdf build failure with texinfo 6.1.0

Hi,

make pdf on Ubuntu 16.04 fail with:

newlib/libc/libc.texinfo:9: Missing @endcsname inserted.

After a lot of fiddling the reason appears to be the combination of concept
and function index despite a lack of concept index entries. Arguably texinfo
should not error in that case but here we are, newlib will fail to build its
documentation on some systems because of this. Since libc.texinfo only
contains function index entries this patch simply removes the combination of
indices. It does the same for libm.texinfo which has concept index entries but
no function index entries.

Tested by running make pdf, make dvi, make info and make html successfully.
libc.pdf appears to have only one index as expected.

== Proposed commit message ==

Fix pdf build failure with texinfo 6.1.0 as provided in Ubuntu 16.04. Index
combination in libc.texinfo and libm.texinfo fails because both file have only
one type of index entries. Removing index combination is thus harmless and
solves the problem.

Is this ok for master?

Best regards,

Thomas