package/collectd: stop passing '--with-libgrpc++'

package/collectd: stop passing '--with-libgrpc++'

Commit 903de16f5fd586a32a7aed7a792a43dce16365eb added passing
'--with-libgrpc++' with the explanation:

    "Use --with-libgrpc++ option as otherwise collectd will try to find
    grpc++.pc which is not available."

At the time of above commit, grpc version in Buildroot was 1.23.0.
Since grpc 1.25.0, a grpc++.pc file _is_ generated from cmake builds.
Hence, remove passing --with-libgrpc++.

This change fixes a problem introduced by commit
fedf3318e3fd3c9ba57389ed2b36472f1a772b9e. As a side effect of that change, a
target version of 'grpc_cpp_plugin' was now created. When collectd was built
after grpc, even without grpc support in collectd enabled, the collectd
configure script would find this target grpc_cpp_plugin and try to use it
(which is not possible because it is built for target).
When not passing '--with-libgrpc++', collectd will instead find the host
version of grpc_cpp_plugin, which works fine.

There are still two underlying problems:
1. the target version of grpc_cpp_plugin is not actually needed. This will
   be disabled in a subsequent commit.

2. collectd should not execute any grpc-related action if grpc support for
   collectd is disabled. This problem has been reported upstream:
   https://github.com/collectd/collectd/issues/3836

Signed-off-by: Thomas De Schampheleire <thomas.de_schampheleire@nokia.com>
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>