delete constraint text

Delete the comment pending incorporation of more details on valid vs. invalid pod configuration values proposed by Damian Conway.

doc/Language/pod.pod6

@@ -68,7 +68,7 @@ Boolean). Also note that "bigints" can be used if required.
 For hashes, '$k1, $k2, ...' are keys of type Str and '$v1, $v2, ...'
 are values of type String, Int, Number, or Boolean.
 
-Strings are delimited by single or double quotes (empty strings are not valid key values). Whitespace is not significant
+Strings are delimited by single or double quotes. Whitespace is not significant
 outside of strings. Hash keys need not be quote-delimited unless they contain
 significant whitespace. Strings entered inside angle brackets become lists if
 any whitespace is used inside the angle brackets.