Make doxygen buildable again

[lurch]

Unfortunately in #2411 @peterharperuk used a doxygen \par keyword which isn't supported by our doxygentoasciidoc repo, and this broke the building of both https://www.raspberrypi.com/documentation/pico-sdk/ and https://datasheets.raspberrypi.com/pico/raspberry-pi-pico-c-sdk.pdf

@mudge It fails with:

....
  File ".../lib/doxygentoasciidoc/nodes.py", line 223, in nodefor
    return {
KeyError: 'title'

Removing these "erroneous" 3 lines from pico_lwip/doc.h is probably easier than fixing up the rest of our doxygen pipeline 😉
And I don't think that this change has too much of an impact on the pico_lwip_http documentation?

(ping also @will-v-pi as he pointed out the problem to me)

[peterharperuk]

Odd, I remember testing this.

[lurch]

Odd, I remember testing this.

Yeah, the plain doxygen build itself works (i.e. make docs from within the CMake build directory), but it's the separate doxygentoasciidoc stage where it fails (see https://github.com/raspberrypi/documentation/blob/develop/CONTRIBUTING.md#build-the-pico-c-sdk-doxygen-documentation )

[mudge]

See also raspberrypi/doxygentoasciidoc#9 which would add support for \par in doxygentoasciidoc.