AST: Remove AvailabilityInference::isAvailableAsSPI().

It has been replaced by `Decl::isAvailableAsSPI()`.

NFC.

Author: tshortli

include/swift/AST/AvailabilityInference.h

@@ -47,9 +47,6 @@ class AvailabilityInference {
   /// Returns the range of platform versions in which the decl is available.
   static AvailabilityRange availableRange(const Decl *D);
 
-  /// Returns true is the declaration is `@_spi_available`.
-  static bool isAvailableAsSPI(const Decl *D);
-
   /// Returns the context for which the declaration
   /// is annotated as available, or None if the declaration
   /// has no availability annotation.

lib/AST/Availability.cpp

@@ -408,7 +408,10 @@ AvailabilityInference::annotatedAvailableRange(const Decl *D) {
 }
 
 bool Decl::isAvailableAsSPI() const {
-  return AvailabilityInference::isAvailableAsSPI(this);
+  if (auto attr = getAvailableAttrForPlatformIntroduction())
+    return attr->isSPI();
+
+  return false;
 }
 
 SemanticAvailableAttributes
@@ -804,13 +807,6 @@ AvailabilityRange AvailabilityInference::availableRange(const Decl *D) {
   return AvailabilityRange::alwaysAvailable();
 }
 
-bool AvailabilityInference::isAvailableAsSPI(const Decl *D) {
-  if (auto attr = D->getAvailableAttrForPlatformIntroduction())
-    return attr->isSPI();
-
-  return false;
-}
-
 std::optional<SemanticAvailableAttr>
 SemanticAvailableAttrRequest::evaluate(swift::Evaluator &evaluator,
                                        const AvailableAttr *attr,