remove unprotected optional deref Errata::severity (#11996)

(cherry picked from commit 775cf94)

lib/swoc/include/swoc/Errata.h

@@ -115,14 +115,9 @@ class Errata {
     /// Check if @a this has a @c Severity.
     bool has_severity() const;
 
-    /// Retrieve the local severity.
-    /// @return The local severity.
-    /// @note The behavior is undefined if there is no local severity.
-    Severity severity() const;
-
     /// Retrieve the local severity.
     /// @return The local severity or @a default_severity if none is set.
-    Severity severity(Severity default_severity) const;
+    Severity severity(Severity default_severity = DEFAULT_SEVERITY) const;
 
     /// Set the @a severity of @a this.
     self_type &assign(Severity severity);
@@ -998,10 +993,6 @@ Errata::Annotation::has_severity() const {
   return _severity.has_value();
 }
 inline auto
-Errata::Annotation::severity() const -> Severity {
-  return *_severity;
-}
-inline auto
 Errata::Annotation::severity(Errata::Severity default_severity) const -> Severity {
   return _severity.value_or(default_severity);
 }