Removed old API checks and some unused code.

We could remove some code as we don't support PHP 5.1 anymore.  I've tested to
build with 5.2.18, 5.3.22, 5.4.12 and 5.5.0 in both ZTS and normal builds at
64bit, as well as 5.3.20 and 5.4.9 in 32bit (no-ZTS).