style.css: Remove

This file is licensed under the GPL. Originally, Nix was also
GPL-licensed so that was fine. However, we later changed the license
to the LGPL but missed the fact that style.css has an incompatible
license.

Since the Nix manual at nixos.org uses its own styling, we can remove
this file.

Fixes #3392.

(cherry picked from commit 9c7e90f)

doc/manual/local.mk

@@ -4,7 +4,6 @@ ifeq ($(doc_generate),yes)
 XSLTPROC = $(xsltproc) --nonet $(xmlflags) \
   --param section.autolabel 1 \
   --param section.label.includes.component.label 1 \
-  --param html.stylesheet \'style.css\' \
   --param xref.with.number.and.title 1 \
   --param toc.section.depth 3 \
   --param admon.style \'\' \
@@ -66,7 +65,7 @@ $(d)/manual.html: $(d)/manual.xml $(MANUAL_SRCS) $(d)/manual.is-valid
 	  $(docbookxsl)/profiling/profile.xsl $< | \
 	  $(XSLTPROC) --output $@ $(docbookxsl)/xhtml/docbook.xsl -
 
-$(foreach file, $(d)/manual.html $(d)/style.css, $(eval $(call install-data-in, $(file), $(docdir)/manual)))
+$(foreach file, $(d)/manual.html, $(eval $(call install-data-in, $(file), $(docdir)/manual)))
 
 $(foreach file, $(wildcard $(d)/figures/*.png), $(eval $(call install-data-in, $(file), $(docdir)/manual/figures)))