[TOP] Remove cscope and tags target from top-level makefile

There are many folks like me who use GUI based editors and
source indexing tools so we should be giving special treatment
to ctags and cscope.

I don't mind having rules in .gitignore to ignore temporary
files generated by indexing tools but having explicit rules
for indexing tools in top-level makefile is not allowed.

One can still have scripts under tools/scripts for assisting
use of his/her favourite indexing tool.

Signed-off-by: Anup Patel <anup@brainfault.org>