Remove unused always-available extension headers

[petk]

These were once used in these files but at this point aren't and are only causing confusion whether file depends on additional extension.

[devnexen]

Looks fine by me

[petk]

Merged to master. I've rechecked it again and all looks ok. The <stdlib.h> is included via php.h and php_config.h. Not ideal and this probably should be enhanced further on, but main point here is for now to only omit including redundant dependencies of these ext headers around the php-src. It makes directory structure a bit easier to understand from the build system point of view. Thanks.