AM::MassAssingmentSecurity: improve performance

[bogdan]

According to this article:
http://merbist.com/2012/02/23/quick-dive-into-ruby-orm-object-initialization/

Current implementation of mass assignment security takes a lot of processor time because spawning to many redundant objects in MassAssignmentSecuritySanitizer#debug_protected_attribute_removal

We can get rid of this method.

Benchmark: https://gist.github.com/2036114

----------when something sanitized----------
New:   0.010000   0.000000   0.010000 (  0.006924)
Old:   0.010000   0.000000   0.010000 (  0.009284)
----------when nothing sanitized----------
New:   0.010000   0.000000   0.010000 (  0.003906)
Old:   0.000000   0.000000   0.000000 (  0.005500) 

A little side effect of this patch attributes are processed one by one. Instead of:

      def process_removed_attributes(attrs)

We get:

      def process_removed_attribute(attr)

The only one place where this could cause side effect is when someone wants to create it's own sanitizer(other than built-in Strict and Logger).

We can make a backward compatibility for this but decided that it doesn't make that much sense.