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

Commit 8d613a1 ("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 <[email protected]>
Signed-off-by: Masahiro Yamada <[email protected]>

Author: ojeda

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.