tarball: remove redundant "version" file

Use the version in Cargo.toml instead.

Print a warning if the Cargo.toml version is not a prefix of the Git
version.  This can happen legit scenarios, see 0dfc490721 (build.rs:
Use Cargo_PKG_VERSION if no version could be found, 2024-06-10)
but the next commit will fix that.

Also remove stale comments in git_version_gen.sh.

build.rs

@@ -35,7 +35,7 @@ fn main() {
     rsconf::set_env_value("BUILD_HOST_TRIPLE", &env_var("HOST").unwrap());
     rsconf::set_env_value("BUILD_PROFILE", &env_var("PROFILE").unwrap());
 
-    let version = &get_version(&env::current_dir().unwrap());
+    let version = &get_version();
     // Per https://doc.rust-lang.org/cargo/reference/build-scripts.html#inputs-to-the-build-script,
     // the source directory is the current working directory of the build script
     rsconf::set_env_value("FISH_BUILD_VERSION", version);
@@ -195,7 +195,7 @@ fn join_if_relative(parent_if_relative: &Path, path: String) -> PathBuf {
     });
 }
 
-fn get_version(src_dir: &Path) -> String {
+fn get_version() -> String {
     use std::fs::read_to_string;
     use std::process::Command;
 
@@ -203,11 +203,6 @@ fn get_version(src_dir: &Path) -> String {
         return var;
     }
 
-    let path = src_dir.join("version");
-    if let Ok(strver) = read_to_string(path) {
-        return strver;
-    }
-
     let args = &["describe", "--always", "--dirty=-dirty"];
     if let Ok(output) = Command::new("git").args(args).output() {
         let rev = String::from_utf8_lossy(&output.stdout).trim().to_string();