kconfig: remove references to docs/misc/kconfig{,-language}.txt files

These files 'docs/misc/kconfig{,-language}.txt' were deleted, but
references are still present in Xen. Remove them to clean-up.

Fixes: 044503f61c ("docs: Delete kconfig docs to fix licensing violation")
Fixes: f80fe2b34f ("xen: Update Kconfig to Linux v5.4")
Signed-off-by: Dmytro Prokopchuk <dmytro_prokopchuk1@epam.com>
Acked-by: Jan Beulich <jbeulich@suse.com>

INSTALL

@@ -23,7 +23,7 @@ Xen Hypervisor
 ==============
 
 Xen itself is configured via a `kconfig' system borrowed from Linux.
-See docs/misc/kconfig.txt.
+See https://www.kernel.org/doc/html/v5.4/kbuild/.
 
 Note that unlike with Linux, and contrary to that document, you cannot
 look at Kconfig files, or the default or generated config files etc.,

MAINTAINERS

@@ -389,7 +389,6 @@ F:	xen/include/xen/iommu.h
 KCONFIG
 M:	Doug Goldstein <cardoe@cardoe.com>
 S:	Supported
-F:	docs/misc/kconfig{,-language}.txt
 F:	xen/tools/kconfig/
 
 KDD DEBUGGER

xen/Kconfig

@@ -1,6 +1,6 @@
 #
 # For a description of the syntax of this configuration file,
-# see docs/misc/kconfig-language.txt
+# see https://www.kernel.org/doc/html/v5.4/kbuild/kconfig-language.html
 #
 mainmenu "Xen/$(SRCARCH) $(XEN_FULLVERSION) Configuration"