bpo-39465: Remove _PyUnicode_ClearStaticStrings() from C API (GH-20078)

Remove the _PyUnicode_ClearStaticStrings() function from the C API.
Make the function fully private (declare it with "static").
diff --git a/Objects/unicodeobject.c b/Objects/unicodeobject.c
index 826298c..34b747e 100644
--- a/Objects/unicodeobject.c
+++ b/Objects/unicodeobject.c
@@ -2289,8 +2289,8 @@
     return id->object;
 }
 
-void
-_PyUnicode_ClearStaticStrings()
+static void
+unicode_clear_static_strings(void)
 {
     _Py_Identifier *tmp, *s = static_strings;
     while (s) {
@@ -16196,7 +16196,7 @@
             Py_CLEAR(unicode_latin1[i]);
         }
 #endif
-        _PyUnicode_ClearStaticStrings();
+        unicode_clear_static_strings();
     }
 
     _PyUnicode_FiniEncodings(tstate);