[3.12] Remove unused SPHINXLINT var from Doc/Makefile. (GH-110570) (#110585)

Remove unused `SPHINXLINT` var from `Doc/Makefile`. (GH-110570)

Remove unused `SPHINXLINT` var.
(cherry picked from commit bdbe43c)

Co-authored-by: Ezio Melotti <[email protected]>

Author: miss-islington

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        =