Remove /.index-build from the .gitignore generated by swift package init

swiftlang/sourcekit-lsp#1803 is moving `.index-build` to be a subdirectory of `.build`.

Author: ahoppen

Sources/Workspace/InitPackage.swift

@@ -433,7 +433,6 @@ public final class InitPackage {
                 """
                 .DS_Store
                 /.build
-                /.index-build
                 /Packages
                 xcuserdata/
                 DerivedData/