Mms: Remove text/plain for ACTION_SEND

Always display registered apps for android.intent.action.SEND action, but this is not true.
Actually, it doesn't support that mime/type. Just remove it to avoid unexpected behaviour for
external apps.

See ComposeMessageActivity#addAttachment

Change-Id: I4815a59533dbfc3decfbdbc57bf456ebda3cab7a
Signed-off-by: Jorge Ruesga <jorge@ruesga.com>