Makefile.uk: Remove redundant check for libunwind

Makefile.uk: Remove redundant check for libunwind

Commit b5b1cf8 (Split off functionality that depends on libunwind)
removed the cyclic dependency between libunwind and libcompiler-rt and
correctly expressed the relationship through Kconfig, making the
build-time check for libunwind in Makefile.uk redundant.
This change removes said redundant check.

Signed-off-by: Andrei Tatar <andrei@unikraft.io>
Reviewed-by: Stefan Jumarea <stefanjumarea02@gmail.com>
Reviewed-by: Eduard Vintilă <eduard.vintila47@gmail.com>
Approved-by: Razvan Deaconescu <razvand@unikraft.io>
Tested-by: Unikraft CI <monkey@unikraft.io>
GitHub-Closes: #19