Remove redundant Ord method impls.

Basically, generic containers should not use the default methods since a
type of elements may not guarantees total order. str could use them
since u8's Ord guarantees total order. Floating point numbers are also
broken with the default methods because of NaN. Thanks for @thestinger.

Timespec also guarantees total order AIUI. I'm unsure whether
extra::semver::Identifier does so I left it alone. Proof needed.

Signed-off-by: OGINO Masanori <masanori.ogino@gmail.com>
auto merge of #8357 : omasanori/rust/cleanup, r=alexcrichton

I feel it's time to eliminate them (and add some testcases.)