docs: removed dead code around FISH_BUILD_VERSION

Man pages used to be built by "build.rs" but now are built by a
dependent "crates/build-man-pages/build.rs". This means that changing
the environment of build.rs is ineffective.

In future, "fn get_version" should probably be a part of
"crates/build-helper/", so Cargo builds only need to compute the
version once.

Lack of this dependency means that "build-man-pages" does not
pass FISH_BUILD_VERSION, which means that Sphinx will fall back to
build_tools/git_version_gen.sh.  This acceptable for now given that
"build-man-pages" is not used in CMake builds.

build.rs

@@ -40,9 +40,6 @@ fn main() {
     // the source directory is the current working directory of the build script
     rsconf::set_env_value("FISH_BUILD_VERSION", version);
 
-    // safety: single-threaded code.
-    unsafe { std::env::set_var("FISH_BUILD_VERSION", version) };
-
     fish_build_helper::rebuild_if_embedded_path_changed("share");
 
     let build = cc::Build::new();

doc_src/conf.py

@@ -122,11 +122,8 @@ if "FISH_BUILD_VERSION_FILE" in os.environ:
     # From Cmake
     f = open(os.environ["FISH_BUILD_VERSION_FILE"], "r")
     ret = f.readline().strip()
-elif "FISH_BUILD_VERSION" in os.environ:
-    # From Cargo
-    ret = os.environ["FISH_BUILD_VERSION"]
 else:
-    # No build system.
+    # From Cargo, or no build system.
     ret = subprocess.check_output(
         ("../build_tools/git_version_gen.sh", "--stdout"), stderr=subprocess.STDOUT
     ).decode("utf-8")