ec: Do not destroy inode context on inode invalidation

Currently there is no need to handle inode invalidation requests,
so this callback has been removed.

This is a backport of http://review.gluster.org/8420/

Change-Id: I0ac2e47679bf62b1493e0403178305923bc036e8
BUG: 1140846
Signed-off-by: Xavier Hernandez <xhernandez@datalab.es>
Reviewed-on: http://review.gluster.org/8702
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Vijay Bellur <vbellur@redhat.com>