CLEANUP: compression: move the default setting of maxzlibmem to defaults

CLEANUP: compression: move the default setting of maxzlibmem to defaults

__comp_fetch_init() only presets the maxzlibmem, and only when both
USE_ZLIB and DEFAULT_MAXZLIBMEM are set. The intent is to preset a
default value to protect the system against excessive memory usage
when no setting is set by the user.

Nowadays the entry in the global struct is always there so there's no
point anymore in passing via a constructor to possibly set this value.
Let's go the cleaner way by always presetting DEFAULT_MAXZLIBMEM to 0
in defaults.h unless these conditions are met, and always assigning it
instead of pre-setting the entry to zero. This is more straightforward
and removes some ifdefs and the last constructor. In addition, now the
setting has a chance of being found.