Revert "An email is a string, not much else." (#373)

This reverts commit 73ef463.

'email' is only a valid type attribute in draft-03 (later versions of
the spec explicitly limit the value of type to the core primitive
types), and this code doesn't validate email addresses anyway. IMO we
should be validating it properly or not at all, and noting this went
away after draft-03 my opinion is on the not-at-all side of the fence.

Note that 'email' is *never* defined as a spec type, in any version -
it just slips in under the radar of the draft-03 language which allows
users to put arbitrary things in the type field.

src/JsonSchema/Constraints/TypeConstraint.php

@@ -206,10 +206,6 @@ protected function validateType(&$value, $type)
             return is_string($value);
         }
 
-        if ('email' === $type) {
-            return is_string($value);
-        }
-
         if ('null' === $type) {
             return is_null($value);
         }