Simplify and speed up del_ebin/1

It is faster and easier to use filename:split/1 and filename:join/1
than use four different 'filename' operations. Each call in the
original call first flattens the input list, and then traverse the
entire string to the end. So there are roughly 8 complete traversals.

In comparison, filename:split/1 will traverse the input string
twice (once to flatten, once to split) and filename:join/1 once.

For background on why this optimization is worthwhile, here is the
result of profiling the start-up of the run-time system on my computer
(done before this optimization):

$ $ERL_TOP/bin/erl -profile_boot
.
.
.
                    filename:join1/4 -  13573 : 13805 us
             erlang:finish_loading/1 -     27 : 19963 us
               filename:do_flatten/2 -  29337 : 49518 us
            erlang:prepare_loading/2 -     49 : 52270 us

Note that filename:do_flatten/2 ends up in second place, almost
as expensive as erlang:prepare_loading/2. Your mileage may vary,
depending on the length of $ERL_TOP and the number of extra
directories added to the path using $ERL_LIBS.

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