Remove the global seccomp option.

Added in af1a9bfb, but it's never been
used in practice because of the performance impact, and it's incompatible
with our ability to add new system calls via a bionic module update
because it defaults to disabling all unknown syscalls, and init will
still be using the non-updated bootstrap libc from when it first shipped.

Test: treehugger
Change-Id: I8e396675fcfaf0218a92f464d15e613f43319305