Drop unnecessary SPI import

Author: DougGregor

lib/ASTGen/Sources/ASTGen/Macros.swift

@@ -1,6 +1,6 @@
 import SwiftParser
 import SwiftSyntax
-@_spi(Testing) import _SwiftSyntaxMacros
+import _SwiftSyntaxMacros
 
 extension SyntaxProtocol {
   func token(at position: AbsolutePosition) -> TokenSyntax? {