store: delete obsolete lsof-disabling code

Since Ifa0adda7984e, we don't use this code anymore on macOS, so we have
no reason to have a knob to disable it anymore.

Change-Id: Ie29a8a8978d9aefd4551895f4f9b3cc0827496df

doc/manual/src/contributing/testing.md

@@ -433,9 +433,6 @@ I grepped `src/` for `get[eE]nv\("` to find the mentions in Lix code.
 - `NIX_PROFILE` - Selects which profile `nix-env` will operate on. Documented elsewhere.
 - `NIX_SSHOPTS` - Options passed to `ssh(1)` when using a ssh remote store.
   Incorrectly documented on `nix-copy-closure` which is *surely* not the only place they are used??
-- `_NIX_TEST_NO_LSOF` - Used on non-Linux, non-macOS platforms to disable using `lsof` when finding gc roots.
-
-  Since https://git.lix.systems/lix-project/lix/issues/156 was fixed, this should probably just be removed as it was a bad workaround for a macOS issue.
 - `_NIX_TEST_GC_SYNC_1` - Path to a pipe that is used to block the GC briefly to validate invariants from the test suite.
 - `_NIX_TEST_GC_SYNC_2` - Path to a pipe that is used to block the GC briefly to validate invariants from the test suite.
 - `_NIX_TEST_FREE_SPACE_FILE` - Path to a file containing a decimal number with the free space that the GC is to believe it has.

src/libstore/gc.cc

@@ -321,10 +321,9 @@ Roots LocalStore::findRoots(bool censor)
 
 void LocalStore::findPlatformRoots(UncheckedRoots & unchecked)
 {
-    // lsof is really slow on OS X. This actually causes the gc-concurrent.sh test to fail.
-    // See: https://github.com/NixOS/nix/issues/3011
-    // Because of this we disable lsof when running the tests.
-    if (getEnv("_NIX_TEST_NO_LSOF") != "1") {
+    // N.B. This is (read: undertested!) fallback code only used for
+    // non-Darwin, non-Linux platforms. Both major platforms have
+    // platform-specific code in src/libstore/platform/
     try {
         std::regex lsofRegex(R"(^n(/.*)$)");
         auto lsofLines =
@@ -337,7 +336,6 @@ void LocalStore::findPlatformRoots(UncheckedRoots & unchecked)
     } catch (ExecError & e) {
         /* lsof not installed, lsof failed */
     }
-    }
 }
 
 void LocalStore::findRuntimeRoots(Roots & roots, bool censor)