Don't use an overlay for src blocks backgrounds

Don't use an overlay for src blocks backgrounds

* org-faces.el (org-block-background): Delete.

* org.el (org-fontify-meta-lines-and-blocks-1): Don't use an
overlay for the block background.
(org-in-src-block-p): Check against the `src-block' text
property, not against an overlay.

The overlay was introduced here:
http://orgmode.org/cgit.cgi/org-mode.git/commit/?id=27d98d879
and fixed here
http://orgmode.org/cgit.cgi/org-mode.git/commit/?id=69811ff44

But (1) it makes fontification slowler, (2) it makes
`org-in-src-block-p' slower and it prevents correct ps-printing
of Org buffers.

If we want to use a different background color, we need another
solution than using overlays.