Remove copy-paste error in wxBase64Encode() documentation

This was apparently accidentally copied from the other overload but doesn't
make sense for this one.

interface/wx/base64.h

@@ -57,10 +57,6 @@ size_t wxBase64Encode(char* dst, size_t dstLen,
 
     There is no error return.
 
-    To allocate the buffer of the correct size, use wxBase64EncodedSize() or
-    call this function with @a dst set to @NULL -- it will then return the
-    necessary buffer size.
-
     @param src
         The input buffer, must not be @NULL.
     @param srcLen