doc: skip running gtkdoc-rebase on local install step

We can leave the relative paths to other books (e.g. '../glib' or
'../ModemManager') because running gtkdoc-rebase seems to be broken
since the 1.29 release, see:

  https://gitlab.gnome.org/GNOME/gtk-doc/-/issues/138

gtk-doc.make

@@ -285,7 +285,6 @@ install-data-local:
 	    mv -f $${installdir}/$(DOC_MODULE).devhelp2 \
 	      $${installdir}/$(DOC_MODULE)-$(DOC_MODULE_VERSION).devhelp2; \
 	  fi; \
-	  $(GTKDOC_REBASE) --relative --dest-dir=$(DESTDIR) --html-dir=$${installdir}; \
 	fi
 
 uninstall-local: