target: remove unused is-builtin

Author: eupn

armv7a-unknown-xous-elf.json

@@ -1,7 +1,6 @@
 {
   "abi": "eabihf",
   "data-layout": "e-m:e-p:32:32-Fi8-i64:64-v128:64:128-a:0:32-n32-S64",
-  "is-builtin": false,
   "linker-flavor": "ld.lld",
   "linker": "rust-lld",
   "llvm-floatabi": "hard",