Fix docs/api sphinx-build err on 'make rpm' (#24334)

The makefile in docs/api/ had a top level check for the
'sphinx-build' tool, but the check was getting triggered
on 'make clean' when it is not needed and causing 'make rpm'
and similar builds to fail.

So this commit removes the check.

Fixes #24163

docs/api/Makefile

@@ -17,11 +17,6 @@ MODULES_PATH  = ../../lib
 EXCLUDE_PATHS = ../../lib/ansible/modules ../../lib/ansible/utils/module_docs_fragments ../../lib/ansible/module_utils/six
 DOC_PROJECTS  = "Ansible API"
 
-# User-friendly check for sphinx-build
-ifeq ($(shell which $(SPHINXBUILD) >/dev/null 2>&1; echo $$?), 1)
-$(error The '$(SPHINXBUILD)' command was not found. Make sure you have Sphinx installed, then set the SPHINXBUILD environment variable to point to the full path of the '$(SPHINXBUILD)' executable. Alternatively you can add the directory with the executable to your PATH. If you don't have Sphinx installed, grab it from http://sphinx-doc.org/)
-endif
-
 # Internal variables.
 PAPEROPT_a4     = -D latex_paper_size=a4
 PAPEROPT_letter = -D latex_paper_size=letter