Remove timing-based signature cache unit test

Two changes:

First removes a unit test that fails in my development environment
(OSX, compiled -g3 with clang).

sipa says that's not terribly surprising; the CMutableTransaction change
makes signing a little more expensive but verification quicker. The unit
test timed sign+verify-uncached versus verify-cached-five-times.

He also says the test will be invalid when libsec256kp1 is integrated
(because validation is super-optimized over signing).

core.h change fixes a compiler warning (clang -Wall : CMutableTransaction defined
as class, declared as struct).