[docs] removed last-modifed line from examples

This is no longer best-practice.

ref:
http://stackoverflow.com/questions/8344186/rails-3-1-on-apache-pagespeed-says-specify-cache-validator-when-using-asset-p/8348385#8348385

railties/guides/source/asset_pipeline.textile

@@ -410,10 +410,6 @@ For Apache:
 
 <plain>
 <LocationMatch "^/assets/.*$">
-  # Some browsers still send conditional-GET requests if there's a
-  # Last-Modified header or an ETag header even if they haven't
-  # reached the expiry date sent in the Expires header.
-  Header unset Last-Modified
   Header unset ETag
   FileETag None
   # RFC says only cache for 1 year
@@ -429,10 +425,6 @@ location ~ ^/assets/ {
   expires 1y;
   add_header Cache-Control public;
 
-  # Some browsers still send conditional-GET requests if there's a
-  # Last-Modified header or an ETag header even if they haven't
-  # reached the expiry date sent in the Expires header.
-  add_header Last-Modified "";
   add_header ETag "";
   break;
 }