Remove hard-coded use of Google Analytics from repository

[strk]

At the moment a couple of makefile targets produce html which use javascript to report page visits to Google. I think we should replace them with some generic way to specify custom scripts to include in the HTML output.

Doing so would simplify the scripts becuase we would not need separate targets for "chunked-html-web" and "chunked-html-web-mobile" as the only difference between those targets and "chunked-html" is the injection of different scripts/css.

It will also be possible to switch between services without the need to change build scripts, for example to use an OSGeo provided analytics service ( see ​https://trac.osgeo.org/osgeo/ticket/2830 )

[Sandro Santilli <strk@…>]

In 04f10ae/git:

Allow passing user flags to xsltproc, drop chunked-html-web target

References #5562

[strk]

There's no more Makefile rule to explicitly use the ga.js file. That file entered the codebase in 2018 (by Paul) and was never modified. We've been using it ONLY for the chunked-html output so far, I don't know where is the data nor if we can download it for reuse in another system, so I'd leave the next steps to whoever knows about those details.

[Sandro Santilli <strk@…>]

In fd46289/git:

Drop google analytics javascript snippet from repository

See ​https://lists.osgeo.org/pipermail/postgis-devel/2023-October/030118.html
Closes #5562