Remove SyntaxProtocol.isActive(in:)

This operation needs to go to the top of the tree to construct a
`ConfiguredRegions` instance before evaluation, making it a performance
trap if called on many nodes. Eliminate it so folks need to form the
`ConfiguredRegions` themselves, which can then be queried efficiently.

Author: DougGregor

Sources/SwiftIfConfig/CMakeLists.txt

@@ -17,7 +17,6 @@ add_swift_syntax_library(SwiftIfConfig
   IfConfigEvaluation.swift
   IfConfigFunctions.swift
   SyntaxLiteralUtils.swift
-  SyntaxProtocol+IfConfig.swift
   VersionTuple+Parsing.swift
   VersionTuple.swift
 )