Remove Unused local variable $block_counter from /core/modules/block/block.module

I've located all four of these variables and have come to the following conclusion.

$block_counter is used as part of the drupal cache process if i've understood the meaning of &drupal_static(__FUNCTION__, array()); correctly.

All instances of $block_id are actually unused, but are (the key) part of a foreach loop. I think removing these won't do the performance of their functions any good, but will reduce code understandability.

In my opinion we should leave these variables untouched.

Because of a discussion in #2057157: Remove Unused local variable $plugin from /core/modules/views/views.module the unused foreach key variables will be removed. I'll supply a patch soon.

Attached patch removes the unused foreach key variables.