Revert "display HTML in default browser using Windows' shell API"

Revert "display HTML in default browser using Windows' shell API"

Since 4804aab (help (Windows): Display HTML in default browser using
Windows' shell API, 2008-07-13), Git for Windows used to call
`ShellExecute()` to launch the default Windows handler for `.html`
files.

The idea was to avoid going through a shell script, for performance
reasons.

However, this change ignores the `help.browser` config setting. Together
with browsing help not being a performance-critical operation, let's
just revert that patch.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>