Remove non-existing file from modulemap.

This picked up a builtin header if it happened to be available.

llvm-svn: 284815

clang/test/Modules/Inputs/libc-libcxx/sysroot/usr/include/module.modulemap

@@ -1,7 +1,6 @@
 module libc [no_undeclared_includes] {
   module math { header "math.h" export * }
   module stdlib { header "stdlib.h" export * }
-  module stdatomic { header "stdatomic.h" export * }
   module stddef { header "stddef.h" export * }
   module stdint { header "stdint.h" export * }
   module stdio { header "stdio.h" export * }