Revert "Default to parallel builds in Sphinx for faster builds"

This reverts commit 309a4382c9267d48fd9d57d0d5372f96c143474c.

Per #3239, it turns out parallel builds use too much RAM for the
average PC out there.

This closes #3239.

Makefile

@@ -2,7 +2,7 @@
 #
 
 # You can set these variables from the command line.
-SPHINXOPTS    = -j auto
+SPHINXOPTS    =
 SPHINXBUILD   = sphinx-build
 PAPER         =
 BUILDDIR      = _build

make.bat

@@ -10,7 +10,6 @@ if "%SPHINXBUILD%" == "" (
 set SOURCEDIR=.
 set BUILDDIR=_build
 set SPHINXPROJ=Test
-set SPHINXOPTS=-j auto
 
 if "%1" == "" goto help