leveldb: disable parallel build

Commit abba4e70 did not succeed in
fixing all parallel build failures because sometimes $(SHARED_OUTDIR) is
created but not $(SHARED_OUTDIR)/db so instead of fixing this mess,
revert the patch and disable parallel build as upstream switched to
cmake

Fixes:
 - http://autobuild.buildroot.org/results/9c33692aa130a20b0f8e868156e49990e862d6ee



Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>