Revert "Only run pip check once per virtual environment"

(This change was broken: pip check did already run once per venv,
but the sub-venv is not always available, so recursion is cleaner.)

This reverts commit 572ca36.

Makefile

@@ -722,10 +722,7 @@ show:
 	@echo OCRD_EXECUTABLES = $(OCRD_EXECUTABLES:$(BIN)/%=%)
 
 check: $(OCRD_EXECUTABLES:%=%-check) $(OCRD_MODULES:%=%-check)
-ifeq (0,$(MAKELEVEL))
 	. $(ACTIVATE_VENV) && pip check
-	. $(SUB_VENV_TF1)/bin/activate && pip check
-endif
 %-check: ;
 
 .PHONY: testcuda test-cuda test-assets test-workflow