build: fix <ndbm.h> detection in AC_PACKAGE_WANT_GDBM

build: fix <ndbm.h> detection in AC_PACKAGE_WANT_GDBM

SUSE systems export the NDBM interface via /usr/include/ndbm.h

This means that dbtest wasn't built on SUSE systems until commit
2353022 (build: update AC_PACKAGE_WANT_GDBM() and src/dbtest.c to
build). That change was incompatible with older SLES releases due to
<ndbm.h> and <gdbm.h> both defining the datum type, resulting in
build failures.

Nothing has ever set HAVE_GDBM_H or checked for the <gdbm.h> header
prior to the above commit, and it's not required on SUSE systems
either, so just remove that entirely.

Signed-off-by: Jeff Mahoney <jeffm@suse.com>
Reviewed-by: Eryu Guan <guaneryu@gmail.com>
Signed-off-by: Eryu Guan <guaneryu@gmail.com>