docs: sphinx-pre-install: don't require the RTD theme

We don't default to the RTD theme anymore, so sphinx-pre-install need not
insist on installing it.

Acked-by: Mauro Carvalho Chehab <[e-mail]>
Signed-off-by: Jonathan Corbet <[e-mail]>

Documentation/sphinx/requirements.txt

···
 # jinja2>=3.1 is not compatible with Sphinx<4.0
 jinja2<3.1
-sphinx_rtd_theme
 Sphinx==2.4.4

scripts/sphinx-pre-install

···
 {
 	my %map = (
 		"python-sphinx"		=> "python3-sphinx",
-		"sphinx_rtd_theme"	=> "python3-sphinx-rtd-theme",
 		"ensurepip"		=> "python3-venv",
 		"virtualenv"		=> "virtualenv",
 		"dot"			=> "graphviz",
···
 {
 	my %map = (
 		"python-sphinx"		=> "python3-sphinx",
-		"sphinx_rtd_theme"	=> "python3-sphinx_rtd_theme",
 		"virtualenv"		=> "python3-virtualenv",
 		"dot"			=> "graphviz",
 		"convert"		=> "ImageMagick",
···
 {
 	my %map = (
 		"python-sphinx"		=> "python3-sphinx",
-		"sphinx_rtd_theme"	=> "python3-sphinx_rtd_theme",
 		"virtualenv"		=> "python3-virtualenv",
 		"dot"			=> "graphviz",
 		"convert"		=> "ImageMagick",
···
 {
 	my %map = (
 		"python-sphinx"		=> "python3-sphinx",
-		"sphinx_rtd_theme"	=> "python3-sphinx_rtd_theme",
 		"virtualenv"		=> "python3-virtualenv",
 		"dot"			=> "graphviz",
 		"convert"		=> "ImageMagick",
···
 sub give_arch_linux_hints()
 {
 	my %map = (
-		"sphinx_rtd_theme"	=> "python-sphinx_rtd_theme",
 		"virtualenv"		=> "python-virtualenv",
 		"dot"			=> "graphviz",
 		"convert"		=> "imagemagick",
···
 sub give_gentoo_hints()
 {
 	my %map = (
-		"sphinx_rtd_theme"	=> "dev-python/sphinx_rtd_theme",
 		"virtualenv"		=> "dev-python/virtualenv",
 		"dot"			=> "media-gfx/graphviz",
 		"convert"		=> "media-gfx/imagemagick",
···
 	$verbose_warn_install = 0;
 
 	add_package("python-sphinx", 0);
-	check_python_module("sphinx_rtd_theme", 1);
 
 	check_distros();
 
···
 	check_perl_module("Pod::Usage", 0);
 	check_program("make", 0);
 	check_program("gcc", 0);
-	check_python_module("sphinx_rtd_theme", 1) if (!$virtualenv);
 	check_program("dot", 1);
 	check_program("convert", 1);