gnutls: remove manual doc disabling kludge

It's no longer required, SUBDIRS += doc is guarded by ENABLE_DOC in
Makefile.am and the only place where it's used regardless is in
dist-hook which isn't used by buildroot.

Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>