kbuild: remove the unneeded comments for external module builds

The supported targets for external modules are listed in the help target
a few lines below. Let's not have duplicated information in two places.
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>

Makefile

@@ -1711,17 +1711,7 @@ else # KBUILD_EXTMOD
 # When building external modules the kernel used as basis is considered
 # read-only, and no consistency checks are made and the make
 # system is not used on the basis kernel. If updates are required
-# in the basis kernel ordinary make commands (without M=...) must
-# be used.
-#
-# The following are the only valid targets when building external
-# modules.
-# make M=dir clean     Delete all automatically generated files
-# make M=dir modules   Make all modules in specified dir
-# make M=dir	       Same as 'make M=dir modules'
-# make M=dir modules_install
-#                      Install the modules built in the module directory
-#                      Assumes install directory is already created
+# in the basis kernel ordinary make commands (without M=...) must be used.
 
 # We are always building only modules.
 KBUILD_BUILTIN :=