docs: kbuild: remove mention to dropped $(objtree) feature

Commit 8d613a1d ("kbuild: drop $(objtree)/ prefix support
for clean-files") dropped support for prefixing with $(objtree).

Thus update the documentation to match that change.
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>

Documentation/kbuild/makefiles.rst

@@ -1042,7 +1042,7 @@ $(clean-files).
 
 When executing "make clean", the file "crc32table.h" will be deleted.
 Kbuild will assume files to be in the same relative directory as the
-Makefile, except if prefixed with $(objtree).
+Makefile.
 
 To exclude certain files or directories from make clean, use the
 $(no-clean-files) variable.