Removed validate.c

As the validate CLI tool has been moved to AppImageUpdate, the outdated version present in this repository has been removed. The readme has been changed as well to link to the correct validate.

Author: Korne127

README.md

@@ -96,7 +96,7 @@ If you invoke an AppImage built with a recent version of AppImageKit with one of
 - `--appimage-mount` mounts the embedded filesystem image and prints the mount point, then waits until it is killed. This is useful if you would like to inspect the contents of an AppImage without executing the contained payload application
 - `--appimage-version` prints the version of AppImageKit, then exits. This is useful if you would like to file issues
 - `--appimage-updateinformation` prints the update information embedded into the AppImage, then exits. This is useful for debugging binary delta updates
-- `--appimage-signature` prints the digital signature embedded into the AppImage, then exits. This is useful for debugging binary delta updates. If you would like to validate the embedded signature, you should use the `validate` command line tool that is part of AppImageKit
+- `--appimage-signature` prints the digital signature embedded into the AppImage, then exits. This is useful for debugging binary delta updates. If you would like to validate the embedded signature, you should use the `validate` command line tool that is part of AppImageUpdate and can be downloaded [here](https://github.com/AppImageCommunity/AppImageUpdate/releases).
 
 ### Special directories