package/protobuf: remove target version of 'protoc'

package/protobuf: remove target version of 'protoc'

The tool 'protoc' and its associated library libprotoc.so are only
needed during development, to convert a protocol buffer definition in the
associated code for a specific code language.

Buildroot does not officially support creating a development environment on
target, so remove these files to reduce disk usage by more than 1.5 MB
(stripped, uncompressed).

Signed-off-by: Thomas De Schampheleire <thomas.de_schampheleire@nokia.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>