Remove --enable-library-evolution flag on prepare. (swiftlang#7635)

(cherry picked from commit d5f6260)

Author: dschaefer2

Sources/Build/BuildDescription/SwiftTargetBuildDescription.swift

@@ -563,7 +563,6 @@ public final class SwiftTargetBuildDescription {
 
         if self.buildParameters.prepareForIndexing {
             args += [
-                "-Xfrontend", "-enable-library-evolution",
                 "-Xfrontend", "-experimental-skip-all-function-bodies",
                 "-Xfrontend", "-experimental-lazy-typecheck",
                 "-Xfrontend", "-experimental-skip-non-exportable-decls",