mailutils: remove crufty workarounds

The AT_TESTED issue has been fixed upstream: "libmu_scm/tests/testsuite.at: Fix improper use of AT_TESTED." in
https://cgit.git.savannah.gnu.org/cgit/mailutils.git/commit/?id=f5a0204d8663b7a583e551647ffbf95b081d39e3 .

The tty issue has been fixed upstream: "comsat/tests/testsuite.at: Use local file instead of the tty." in
https://cgit.git.savannah.gnu.org/cgit/mailutils.git/commit/?id=016c4a978a69cc8990b5aab6c88cbac6fda57203 .

The LD_LIBRARY_PATH has been added in 2017 and seems to be no longer necessary as the test suite passes without it.

pkgs/tools/networking/mailutils/default.nix

@@ -112,20 +112,6 @@ stdenv.mkDerivation (finalAttrs: {
   ];
 
   doCheck = !stdenv.hostPlatform.isDarwin; # ERROR: All 46 tests were run, 46 failed unexpectedly.
-  doInstallCheck = false; # fails
-
-  preCheck = ''
-    # Disable comsat tests that fail without tty in the sandbox.
-    tty -s || echo > comsat/tests/testsuite.at
-    # Remove broken macro
-    sed -i '/AT_TESTED/d' libmu_scm/tests/testsuite.at
-    # Provide libraries for mhn.
-    export LD_LIBRARY_PATH=$(pwd)/lib/.libs
-  '';
-
-  postCheck = ''
-    unset LD_LIBRARY_PATH
-  '';
 
   meta = {
     description = "Rich and powerful protocol-independent mail framework";