Stop exporting isassigned() and normalize() from Unicode

Since these conflict with functions from Base, they cannot be called
without the module name anyway, and they make it impossible to call the
functions from Base once 'using Unicode' has been called.

stdlib/Unicode/src/Unicode.jl

@@ -9,7 +9,7 @@ using Base.Unicode: normalize, graphemes, isassigned, textwidth, isvalid,
                     iscntrl, ispunct, isspace, isprint, isgraph,
                     lowercase, uppercase, titlecase, lcfirst, ucfirst
 
-export normalize, graphemes, isassigned, textwidth, isvalid,
+export graphemes, textwidth, isvalid,
        islower, isupper, isalpha, isdigit, isxdigit, isnumeric, isalnum,
        iscntrl, ispunct, isspace, isprint, isgraph,
        lowercase, uppercase, titlecase, lcfirst, ucfirst