expr: remove redundant about text from after-help

[l46983284-cpu]

Fixes #13983.

The expr utility showed its about text twice in --help output:
once from the about field and again at the start of after-help.

Remove the duplicated first sentence from expr-after-help in both
en-US and fr-FR locales so the help text reads naturally.

Before:

Print the value of EXPRESSION to standard output

Usage: expr [EXPRESSION]
       expr [OPTIONS]
...

Print the value of EXPRESSION to standard output. A blank line below
separates increasing precedence groups.

After:

Print the value of EXPRESSION to standard output

Usage: expr [EXPRESSION]
       expr [OPTIONS]
...

A blank line below
separates increasing precedence groups.

[github-actions]

GNU testsuite comparison:

GNU test failed: tests/cat/splice. tests/cat/splice is passing on 'main'. Maybe you have to rebase?
GNU test failed: tests/misc/empty-backup-suffix. tests/misc/empty-backup-suffix is passing on 'main'. Maybe you have to rebase?
GNU test failed: tests/misc/uname-labeled. tests/misc/uname-labeled is passing on 'main'. Maybe you have to rebase?

[mustafaelrasheid]

I think it would be better if there was no awkward line break.
Something like

A blank line below
separates increasing precedence groups.

should typically look like

A blank line below separates increasing precedence groups.

since the sentence doesn't pass over 80 characters.

[l46983284-cpu]

I kept the line break to mirror GNU coreutils style. GNU expr --help has the same break:

A blank line below
separates increasing precedence groups.

Parity with GNU is the goal here.

[l46983284-cpu]

CI failures are unrelated to this change. The 3 failing GNU tests are in the backport list in util/fetch-gnu.sh — fetched at CI time via curl from raw.githubusercontent.com. Logs show the scripts came down as HTML error pages instead of shell. None of them reference expr. Transient GitHub flake.