Fix crashes when running in debug on OSX

When building debug-and-release, only the release version of
core_generated.gyp, defining NDEBUG, would be generated by
gyp_generator.prf.

Both the debug and release ninja files would be generated by gyp from
that same file and we would end up defining NDEBUG for src/core
source files even in debug while the rest of the content layer would
be properly built without it, causing crashes with object having
unexpected sizes and non-symetrical clean-up routines.

Skia doesn't seem to complain anymore if I build in release without
this so simply remove it.

Change-Id: I021f87f2dac5aa21831992824d39dc44216b5f58
Reviewed-by: Andras Becsi <andras.becsi@digia.com>