rpcgen: Remove autogenerated files instead build on demand

Avoid modifying autogenerated files and keeping them in
repository - autogenerate them on demand from ".x" files

Change-Id: I2cdb1fe9b99768ceb80a8cb100fa00bd1d8fe2c6
BUG: 1090807
Signed-off-by: Harshavardhana <harsha@harshavardhana.net>
Reviewed-on: http://review.gluster.org/7526
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Niels de Vos <ndevos@redhat.com>
Reviewed-by: Anand Avati <avati@redhat.com>