stdenv: linux: drop dead dejagnu.doCheck=false override from stage2

dejagnu is never demand-built in the bootstrap closure (verified via
`nix-store --query --requisites | grep dejagnu` returning empty on
master). The override existed but the resulting derivation was never
referenced by anything stage2+ actually builds.

Verified by drv-hash equality: stdenv.drvPath is unchanged
(q2xn5647kadsgpz40xcmjssa0pmdmiwi-stdenv-linux.drv).

pkgs/stdenv/linux/default.nix

@@ -458,9 +458,6 @@ in
           texinfo
           which
           ;
-        dejagnu = super.dejagnu.overrideAttrs (a: {
-          doCheck = false;
-        });
 
         # Avoids infinite recursion, as this is in the build-time dependencies of libc.
         libiconv = self.libcIconv prevStage.libc;