[libc++] Remove empty container benchmarks that are meaningless from the associative containers (#175180)
We generally try to benchmark the empty container case in the
associcative containers, since there are some case where we previously
ran calculations for significant amounts of time, even though they were
really not needed. However, there are some benchmarks where trying to
run with an empty container doesn't make sense (e.g. when benchmarking
collisions). We've worked around this a bit in the benchmarks by not
actually benchmarking the empty case. This patch removes these
meaningless benchmarks instead to speed up running the benchmarks.