dumpstate: remove the arbitrary limit on tombstones.

Don't just arbitrarily limit to 10. Assume the user knows what they're
doing (or is at least happy with the trade-off).

The initial idea was to just honor debuggerd's system property for how
many tombstones to collect (because why tell a device to collect N
tombstones if you don't also want it to give you N tombstones?), but
then why duplicate that check here? Instead we assume that anyone who's
manually decreasing the system property is also going to wipe their
tombstones directory.

Bug: http://b/140580637
Test: run crasher, dumpstate
Change-Id: Iad5f5a3a4c9d7d089514f4a02b247d83ececfa90