db: drop legacy ARM specific SWP based mutex optimisation

Although the ARM SWP instruction may exist for ARMv6 and above, it's
not guaranteed to work, especially on SMP systems where it's use may
lead to instability at runtime, etc:

  https://community.arm.com/processors/b/blog/posts/locks-swps-and-two-smoking-barriers

Keeping the optimisation for architecture levels which pre-date SMP
(ie <= ARMv5) may be safe, however other distros (Buildroot, Debian,
Fedora, etc) are not doing so and mutex contention is likely to be
less of an issue on uniprocessor systems anyway, so the benefits of
this micro optimisations are not clear. Since OE uses ARMv5 qemu as
a proxy for testing all 32bit ARM architecture levels, it's desirable
to keep the ARMv5 builds aligned with later ARM architecture levels
wherever possible.

Signed-off-by: Andre McCurdy <armccurdy@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

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