[lld/tests] Stop setting the "asserts" and "debug" features

The last use of `REQUIRES: debug` was removed in 2013 in 72c5d3d7 in favor of
`REQUIRES: asserts`.

The last use of `REQUIRES: asserts` was removed in 2015 in 251b0e26 when the
old COFF linker was removed.

lld's test suite currently has no behavior difference with respect to
assertions or debug builds (and hasn't had it for 6 years). Let's keep it that
way :)

Differential Revision: https://reviews.llvm.org/D115941