open5gs: remove extra compiler flags

Seems to be no longer needed with GCC and additionally breaks the
build with Clang.

pkgs/by-name/op/open5gs/package.nix

···
     usrsctp
   ];
 
-  # For subproject
-  env = {
-    NIX_CFLAGS_COMPILE = toString [
-      "-Wno-error=array-bounds"
-      "-Wno-error=stringop-overflow"
-    ];
-  };
-
   preConfigure = ''
     cp -R --no-preserve=mode,ownership ${finalAttrs.diameter} subprojects/freeDiameter
     cp -R --no-preserve=mode,ownership ${finalAttrs.libtins} subprojects/libtins