Remove @_spi(ExperimentalLanguageFeature) from BodyMacro

Function body macros have been accepted.

Author: ahoppen

Sources/SwiftSyntaxMacros/MacroProtocols/BodyMacro.swift

@@ -16,7 +16,6 @@ import SwiftSyntax
 
 /// Describes a macro that can create the body for a function that does not
 /// have one.
-@_spi(ExperimentalLanguageFeature)
 public protocol BodyMacro: AttachedMacro {
   /// Expand a macro described by the given custom attribute and
   /// attached to the given declaration and evaluated within a