build_tools/release.sh: docs have been removed from tarball

See 135fc73191 (Remove man/HTML docs from tarball, require Sphinx
instead, 2025-11-20).

doc_src/conf.py

@@ -129,10 +129,6 @@ else:
         ("../build_tools/git_version_gen.sh", "--stdout"), stderr=subprocess.STDOUT
     ).decode("utf-8")
 
-predetermined_timestamp = os.environ.get("FISH_SPHINX_BUILD_DATE")
-if predetermined_timestamp:
-    today = predetermined_timestamp
-
 # The full version, including alpha/beta/rc tags
 release = ret.strip().split(" ")[-1]
 # The short X.Y version