Remove sequential search from DwarfEhFrameWithHdr.

Since there has never been a time that a sequential search has been
necessary, simply remove the sequential code and allow the DwarfEhFrame
object to be used in that case. If this does come up regularly, I'm not
sure that it wouldn't be more performant to use the DwarfEhFrame object
regardless.

Bug: 129550015

Test: libunwindstack unit tests, libbacktrace unit tests.
Change-Id: I738e81accb9bc1e182168cddc9c0c22823e3369b