Remove unnecessary clause about validation. (#213)

The spec doesn't define what it means to validate the currency string so
it is impossible to know what NOT to do. Additionally, specs should
describe what _should_ be done and not what should not be done in order
to achieve the desired outcome.

Closes #175.

index.html

@@ -645,8 +645,7 @@ <h2>PaymentCurrencyAmount</h2>
     <dd>
       <code>currency</code> is a string containing a currency identifier. The most common
       identifiers are three-letter alphabetic codes as defined by [[!ISO4217]] (for example,
-      <code>"USD"</code> for US Dollars) however any string is considered valid and
-      <a data-lt="user agents">user agents</a> MUST not attempt to validate this string.
+      <code>"USD"</code> for US Dollars) however any string is considered valid.
     </dd>
     <dt><code><dfn>value</dfn></code></dt>
     <dd>