Revert "Add conversion from regex syntax to BNF syntax (#803)"

This reverts commit 7b33d61.

Author: milseman

Package.swift

@@ -135,13 +135,6 @@ let package = Package(
                 "_RegexParser",
                 "_StringProcessing"
             ]),
-        .executableTarget(
-            name: "Regex2BNF",
-            dependencies: [
-                .product(name: "ArgumentParser", package: "swift-argument-parser"),
-                "_RegexParser"
-            ],
-            swiftSettings: [availabilityDefinition]),
         .executableTarget(
             name: "RegexTester",
             dependencies: [