Do not delete include directory.

This avoids fed-gen race condition. It will lead to quadratically many
file writes in CI. It also allows users to put their own files in
the include directory if they like. I am not sure if this is a "feature"
that we actually want. We can discuss later.

org.lflang/src/org/lflang/generator/c/CGenerator.java

@@ -946,7 +946,7 @@ private void generateReactorDefinitions() throws IOException {
     }
 
     private void generateHeaders() throws IOException {
-        FileUtil.deleteDirectory(fileConfig.getIncludePath());
+        // Cannot delete existing header files directory because this would create a race condition in fed-gen
         FileUtil.copyDirectoryFromClassPath(
             fileConfig.getRuntimeIncludePath(),
             fileConfig.getIncludePath(),