kbuild: remove -I$(srctree)/tools/include from scripts/Makefile

I do not like to add an extra include path for every tool with no
good reason. This should be specified per file.

This line was added by commit 6520fe55

 ("x86, realmode: 16-bit
real-mode code support for relocs tool"), which did not touch
anything else in scripts/. I see no reason to add this.

Also, remove the comment about kallsyms because we do not have any
for the rest of programs.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>