Revert "kvm tools: Add kvm__has_cap() to check whether a cap is available on the host"

We already had kvm__has_extension, no need for two functions that do the
same.

This reverts commit 42efb1abf4ebebeedd14af34c073e673923e2898.

Conflicts:

	tools/kvm/include/kvm/kvm.h

Signed-off-by: Sasha Levin <levinsasha928@gmail.com>
Signed-off-by: Pekka Enberg <penberg@kernel.org>