remove need for doxygen's PROJECT_NUMBER manipulation

[trel]

The current docker builder always removes and re-clones the requested iRODS repository - so that it is always a clean build and honors any upstream changes in the iRODS codebase.

However, this also means that getting Doxygen's PROJECT_NUMBER to render correctly requires commenting out this 'removal' code in the Makefile, updating the Doxyfile, and then running the docker builder again.

irods_docs/Makefile

Lines 23 to 24 in fe00182

	@if [ -d ${IRODSTARGET} ] ; then rm -rf ${IRODSTARGET} ; fi
	@git clone https://github.com/${GITHUB_ACCOUNT}/irods ${IRODSTARGET}

Updating the Doxyfile directly will keep the current workflow available and render correctly the requested GIT_COMMITTISH of the code.