Remove mentions of Niecza and incorrect Rakudo reference per #220

doc/Type/Str.pod

@@ -71,8 +71,6 @@ Does a Unicode "fold case" operation suitable for doing caseless
 string comparisons.  (In general, the returned string is unlikely to
 be useful for any purpose other than comparison.)
 
-(Not implemented in Rakudo and Niecza)
-
 =head2 routine tc
 
     multi sub    tc(Str:D ) returns Str:D
@@ -82,8 +80,6 @@ Does a Unicode "titlecase" operation, that is changes the first character in
 the string to title case, or to upper case if the character has no title case
 mapping
 
-(Not implemented in Niecza)
-
 =head2 routine tclc
 
     multi sub    tclc(Str:D ) returns Str:D
@@ -92,8 +88,6 @@ mapping
 Turns the first character to title case, and all other characters to lower
 case
 
-(not implemented in Niecza)
-
 =head2 routine wordcase
 
 =for code :allow<L>