Don't handle unknown_symbols in \operatorname.

Explicitly listing unknown_symbol (i.e., `\nonexistingsymbol`) in the
`\operatorname` mathtext parser doesn't really matter as the parse
action for such symbols is to immediately throw an exception; so we may
as well not bother listing them.

Author: anntzer

lib/matplotlib/_mathtext.py

@@ -1881,8 +1881,7 @@ def csnames(group, names):
 
         p.function <<= csnames("name", self._function_names)
         p.operatorname <<= cmd(
-            r"\operatorname",
-            "{" + ZeroOrMore(p.simple | p.unknown_symbol)("name") + "}")
+            r"\operatorname", "{" + ZeroOrMore(p.simple)("name") + "}")
 
         p.group <<= p.start_group + ZeroOrMore(p.token)("group") + p.end_group