gnutls: drop wchar dependency

gnutls: drop wchar dependency

Commit 19448f40a0 (gnutls: use included unistring unless libunistring is
selected) made libunistring an optional dependency.  So now gnutls no longer
depend on wchar.

Drop wchar dependency of libmicrohttpd and libsoup, which are gnutls
reverse dependencies that do not depend themselves on wchar.

Update the comments in libsoup and taskd; the wchar dependency is now
not due to gnutls.

Cc: Matt Weber <matthew.weber@rockwellcollins.com>
Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>