Fixed warning unused no-compiler-rt

The
  warning: target json file contains unused fields: no-compiler-rt
is now gone.

Author: stappersg

avr-atmega328p.json

@@ -8,7 +8,6 @@
   "linker-flavor": "gcc",
   "linker-is-gnu": true,
   "llvm-target": "avr-unknown-unknown",
-  "no-compiler-rt": true,
   "os": "unknown",
   "position-independent-executables": false,
   "exe-suffix": ".elf",