QRegExpAutomatonState: fix Coverity complaint about uninit'ed data members

Coverity rightfully complained that `atom` and `match` were left
uninit'ed by the class' default ctor.

That ctor is never called in Qt 6, though, and only present because
Q5Vector used to require a default constructor for mere
realloc'ing. So fix the Coverity complaint by removing the ctor.

Amends the start of the public history (the code could have just used
std::vector instead already then, which never had that problem).

Pick-to: 6.12 6.11 6.8
Coverity-Id: 11602
Change-Id: Iac34131256b2489fb7214696ac647bd9555ce6d6
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>