Remove unused SPHINXLINT var from Doc/Makefile. (#110570)

Remove unused `SPHINXLINT` var.

Author: ezio-melotti

Doc/Makefile

@@ -7,7 +7,6 @@
 PYTHON       = python3
 VENVDIR      = ./venv
 SPHINXBUILD  = PATH=$(VENVDIR)/bin:$$PATH sphinx-build
-SPHINXLINT   = PATH=$(VENVDIR)/bin:$$PATH sphinx-lint
 BLURB        = PATH=$(VENVDIR)/bin:$$PATH blurb
 JOBS         = auto
 PAPER        =