Do not delete source files when debugging

[yyny]

[tianon]

This "oh, but one more thing" is actually part of the reason why we resisted implementing DOCKER_PHP_ENABLE_DEBUG for so long :sweat_smile: :heart:

In this specific case, this is really trivial to implement via docker-php-source extract post-build, so without a stronger justification than "it'd be handy", I'm afraid this one is a likely decline. :bow:

[mvorisek]

I support this PR. When debug build is enabled, it does not make any sense to remove the sources. I understand the post from @tianon, but let's make developers live not harder.

[yyny]

docker-php-source extract wouldn't quite work for me because I am patching the sources.